Give a contributor a way in

There was none. 14 documents, 177 numbered requirements, and all of it
written for someone who has already decided to work on this — nothing that
tells a newcomer which door is unlocked, what a first build needs, or why it
takes so long.

CONTRIBUTING.md points the first door at the operation format, because
"add a develop node" is a genuinely one-file contribution and the best first
experience this codebase can offer: no Rust, no shader edit, no UI change,
and its tests declared in the same file. It teaches the architecture's
central idea on the way through, which is why the invariant test added in
the previous commit is named there rather than left to be discovered.

Three things that were folklore are now written down: Git LFS is a
prerequisite, the first build resolves 826 crates and is not hanging, and
Slint needs pkg-config, libfontconfig1-dev and libxkbcommon-dev. The LFS one
proved itself while writing this — a fresh worktree hit exactly the failure
`dr-segment`'s build script is written to catch, which is the argument for
saying so before it happens rather than after.

rust-toolchain.toml pins 1.92.0 because `build-and-test.yml` already does
and says why: a floating toolchain turns an unrelated push into a mystery
failure. The two checks that gate every push are the two most sensitive to
compiler version — rustfmt's output changes between releases, so a
contributor on a newer stable can produce a diff nobody wrote on a line
nobody touched, and `clippy -D warnings` is the same story with new lints.
`rust-version = "1.92"` in the manifest stays where it is; it is a minimum,
and this is the upper bound it cannot express.

Also states the convention the tooling cannot enforce, from code-health.md
CH-4: close a requirement with a test that would fail if the behaviour were
removed. Coverage that moves slowly and means something beats coverage that
moves quickly.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CONTRIBUTING.md

@@ -0,0 +1,163 @@
+# Contributing to DarkRoom
+
+There is a lot of documentation here — 14 documents and 177 numbered
+requirements — and almost all of it is written for someone who has already
+decided to work on this. This file is the other thing: how to get a first
+change landed without reading any of it.
+
+## The shortest useful contribution
+
+**A develop operation is one file.** Not one file plus a registration, plus a
+shader edit, plus a control in the UI — one file:
+
+```
+core/dr-pipeline/ops/split_toning.yaml
+```
+
+`build.rs` finds it with `read_dir`, compiles it into Rust implementing
+`Operation`, and from there it is indistinguishable from a hand-written node.
+It arrives with controls built from its declared parameter kinds, a place in
+the chain from `order:`, a place in the panel from `attributes:`, sidecar
+persistence, and its own tests — which are declared in the same file and run
+under `cargo test`.
+
+Read [`core/dr-pipeline/ops/README.md`](core/dr-pipeline/ops/README.md) and
+copy [`exposure.yaml`](core/dr-pipeline/ops/exposure.yaml). Split toning,
+colour zones, selective colour and channel-mixer variants are all pure point
+operations, which means all of them are declarations rather than code.
+
+If you want to understand one thing about the architecture before starting,
+make it this: **the core describes its capabilities and the interface composes
+them.** No code in `ui/` names an operation, and a test enforces that
+(`ui/dr-ui/tests/ui_names_no_operation.rs`). It is why your node needs no UI
+change.
+
+## Getting it to build
+
+**Git LFS is required.** Model weights are stored in LFS, and a clone made
+without it leaves a ~130-byte text pointer where an 11 MB model should be:
+
+```bash
+git lfs install && git lfs pull
+```
+
+Forget this and `dr-segment`'s build script stops with an instruction rather
+than embedding the pointer and failing at inference time — but it is easier to
+run the two commands now.
+
+**The toolchain pins itself.** `rust-toolchain.toml` selects 1.92.0 and rustup
+fetches it on first use. Do not override it; `cargo fmt` and `clippy` are both
+version-sensitive and CI runs exactly this version.
+
+**System packages.** Slint and winit need these at build time. On Debian or
+Ubuntu:
+
+```bash
+sudo apt-get install pkg-config libfontconfig1-dev libxkbcommon-dev
+```
+
+**Then:**
+
+```bash
+cargo run -p darkroom-desktop
+```
+
+The first build resolves 826 crates and takes a while — on a laptop, long
+enough to look like a hang. It is not one.
+
+Android is a containerised toolchain and is not needed for most work; see
+[`docker/android/README.md`](docker/android/README.md) if you get there.
+
+## What CI will check
+
+All four of these run on every push, so run them before you send anything:
+
+```bash
+cargo fmt --all -- --check
+cargo clippy --workspace --all-targets -- -D warnings
+cargo test --workspace
+cargo build --workspace --release
+```
+
+GPU tests skip themselves where there is no adapter rather than failing — a
+test that cannot run is not evidence either way — so a green run on a machine
+without a GPU is expected, and does not mean the GPU paths were exercised.
+
+## Requirements and traceability
+
+[`requirements.md`](docs/requirements.md) is the register of record.
+[`traceability.md`](docs/traceability.md) is generated from `TRACES:` tags in
+the source and must never be hand-edited:
+
+```rust
+// TRACES: FR-DEV-3a | FR-DEV-3c
+```
+
+Tags are read from `.rs`, `.slint`, `.wgsl` and `.yaml` — the last so a
+declared operation can record the requirement it satisfies, since the Rust it
+generates lands in `OUT_DIR` and is not scanned.
+
+A pre-commit hook regenerates the matrix and stages it whenever you touch
+something that can carry a tag, so you should not have to think about it. If
+you do need to run it by hand:
+
+```bash
+cargo run -p traceability -- report
+```
+
+Note that it tracks line numbers, so a change that only moves code still moves
+the matrix. Never regenerate it with a stale prebuilt binary.
+
+**One convention that the tooling cannot enforce.** A tag proves that a tag
+exists, not that the code under it does the thing — `docs/code-health.md`
+CH-4 has the details, and two requirements currently read as covered on the
+strength of plumbing a future feature would use. So: **close a requirement
+with a test that would fail if the behaviour were removed.** Coverage that
+moves slowly and means something beats coverage that moves quickly.
+
+## Two invariants the build defends
+
+Worth knowing before you trip one, because both failures name a requirement
+rather than a line:
+
+- **No operation may be named in `ui/`** (FR-DEV-3a). Special-casing one
+  operation in the panel to fix a layout problem is how a generated interface
+  stops being generated. If a node needs presentation the panel cannot give it,
+  the answer is a `presentation:` hint in the declaration and a `WidgetKind`,
+  not a branch in `develop.rs`.
+- **The operation schema rejects ambiguity at build time**: a duplicate
+  `order:`, a filename disagreeing with its `id:`, a default outside its own
+  range, an expression naming something that is not a parameter. Each error
+  names the key you got wrong and exits rather than panicking.
+
+## Commit messages
+
+Imperative subject describing the change from the reader's side — "Offer the
+merge when two people turn out to share a name", not "fix: merge dialog". No
+conventional-commits prefixes.
+
+The body is where the reasoning goes, and it is expected to be substantial when
+the change is. This codebase records *why* far more than most, in commits and
+in comments alike, and that is the single habit most worth adopting: the
+constraint you worked around is invisible to whoever reads the diff next.
+
+One commit per change. If you fixed two things, that is two commits.
+
+## Where to read next, in order
+
+| Document | Read it when |
+|---|---|
+| [`core/dr-pipeline/ops/README.md`](core/dr-pipeline/ops/README.md) | Adding or changing a develop operation — start here regardless |
+| [`docs/architecture.md`](docs/architecture.md) | Anything touching the render path, catalog or sync |
+| [`docs/code-health.md`](docs/code-health.md) | Deciding what to work on; grades each seam by what it costs |
+| [`docs/technical-debt.md`](docs/technical-debt.md) | Something looks wrong — check it was not chosen |
+| [`docs/requirements.md`](docs/requirements.md) | Reference, not reading |
+
+`technical-debt.md` is the one to check before "fixing" anything surprising.
+It records compromises that were deliberate, each with the reasoning and a
+falsifiable condition for when it stops being one — the point being that you
+can tell a constraint from an accident without asking.
+
+## Licence
+
+GPL-3.0-or-later. By contributing you agree your work is licensed the same way.