Put the developer docs under docs/dev and index the folder for users first

docs/ had 26 developer documents flat beside the manual, and the two
audiences are very differently sized: most readers want the manual and
the gesture reference, a few want the register, the designs and the
measurements. The manual and gestures.md stay at the top; everything for
someone changing the code moves to docs/dev/, and the two documents that
name their own successors — the v0.1 milestone and the UI-refinement plan
— go to docs/dev/archive/ rather than being deleted, since both are still
cited. docs/README.md is the index, users first.

Every reference follows: code comments, Cargo manifests, the workflows,
the pre-commit hook, the bench and traceability tools (which locate the
repo root by docs/dev/requirements.md now), packaging, the Docker READMEs,
CLAUDE.md, CONTRIBUTING.md and the README. The matrix links one level
deeper and is regenerated. Links out of the moved documents into the tree
gain a level; a link checker over every Markdown file finds none broken.

.gitea/workflows/benchmark.yml

@@ -1,6 +1,6 @@
 name: Benchmarks
 
-# The suite docs/requirements.md §8 has been promising since it was written:
+# The suite docs/dev/requirements.md §8 has been promising since it was written:
 # "an automated benchmark suite against a synthetic 50k catalog, run per-commit
 # … A regression beyond stated tolerance fails the build."
 #
@@ -29,7 +29,7 @@ name: Benchmarks
 #         every commit to establish, every time, that this runner has no GPU. It
 #         runs on demand (Actions → Run workflow) so that a runner that *does*
 #         have one can be pointed at it, and the numbers it produces belong in
-#         docs/frame-budget.md by hand, as they already are.
+#         docs/dev/frame-budget.md by hand, as they already are.
 
 on:
   push:
@@ -183,7 +183,7 @@ jobs:
       - name: Frame budget (FR-DSP-3)
         run: cargo test --release -p dr-gpu --test frame_budget -- --nocapture
 
-      # The instrument behind docs/frame-budget.md. It exits non-zero with no
+      # The instrument behind docs/dev/frame-budget.md. It exits non-zero with no
       # adapter, which is right for a tool a person runs deliberately and wrong
       # for a job that usually has none — hence continue-on-error. Its table is
       # in the log for whoever asked for this run; the committed numbers are

.gitea/workflows/build-and-test.yml

@@ -322,7 +322,7 @@ jobs:
         env:
           CARGO_TARGET_DIR: target-android
           # Absent secrets mean a debug signature, which is what a fork or a
-          # branch build should get. Set all three (see docs/android-signing.md)
+          # branch build should get. Set all three (see docs/dev/android-signing.md)
           # and the same job produces a release-signed APK instead.
           ANDROID_KEYSTORE_BASE64: ${{ secrets.ANDROID_KEYSTORE_BASE64 }}
           KEYSTORE_PASS: ${{ secrets.ANDROID_KEYSTORE_PASSWORD }}
@@ -370,7 +370,7 @@ jobs:
 
   # TRACES: FR-PLAT-WIN-3
   # The Windows executable and its installer, cross-built from Linux
-  # (docs/windows.md §7). No Windows machine anywhere in this job: what it
+  # (docs/dev/windows.md §7). No Windows machine anywhere in this job: what it
   # can prove is that the binary links, is a Windows executable with no
   # MinGW runtime imports, starts under Wine, and that the installer installs
   # and uninstalls under Wine. What it cannot prove — a Vulkan device, a

.gitea/workflows/traceability-check.yml

@@ -7,7 +7,7 @@ name: Traceability
 # fail its own threshold. Two rules follow, and the extractor's own tests
 # enforce both:
 #
-#   1. Denominators are parsed from docs/requirements.md at run time.
+#   1. Denominators are parsed from docs/dev/requirements.md at run time.
 #   2. Coverage is |traced ∩ defined| / |defined|, never a raw traced count.
 #
 # This job is static analysis of source comments plus markdown parsing, so it
@@ -74,11 +74,11 @@ jobs:
         run: |
           set -e
           cargo run -q -p traceability -- report
-          if ! git diff --quiet docs/traceability.md; then
+          if ! git diff --quiet docs/dev/traceability.md; then
             echo ""
-            echo "docs/traceability.md is out of date."
+            echo "docs/dev/traceability.md is out of date."
             echo "Run: cargo run -p traceability -- report"
-            git diff --stat docs/traceability.md
+            git diff --stat docs/dev/traceability.md
             exit 1
           fi
 
@@ -127,4 +127,4 @@ jobs:
 
       - name: Summary
         if: always()
-        run: head -30 docs/traceability.md || true
+        run: head -30 docs/dev/traceability.md || true