Add requirements traceability gate and Gitea pipelines

Ports JellyTau's traceability tooling to Rust, carrying across the bug it
was repaired for. That gate divided a traced count by frozen literal
denominators; the requirements file outgrew them and it reported 158%
coverage, so it could never fail its own threshold.

Two rules, both enforced by the extractor's own tests:

  - denominators parsed from docs/requirements.md at run time
  - coverage is |traced ∩ defined| / |defined|, never a raw traced count

The gate additionally fails hard on a misconfigured run — zero
requirements parsed or zero files scanned — rather than reporting a
plausible 0%, and on any orphan tag naming a requirement that does not
exist.

Adapted for DarkRoom: IDs are FR-CAT-1 / NFR-P13 / FR-DEV-3a shapes
rather than JellyTau's fixed three digits, and decisions (D), spikes (S),
milestone items (M) and test ids remain taggable while being excluded
from the denominator — counting them inflated it by 25.

Also adds dr-sync: the RemoteBackend trait and capability model, so the
Nextcloud connector is one implementation rather than the only shape the
engine understands. No mature Nextcloud crate exists (reqwest_dav is too
thin), so the connector will be hand-rolled over reqwest per D7.

Gitea workflows follow the same style: containerised, commented with the
reasoning, desktop and Android on every push, plus a CI check that no
core/ crate depends on the UI toolkit (ARCH §6.5a).

Coverage today: 13.3% (19/143). 50 tests passing.

Cargo.lock

@@ -1102,6 +1102,16 @@ dependencies = [
  "wgpu",
 ]
 
+[[package]]
+name = "dr-sync"
+version = "0.1.0"
+dependencies = [
+ "async-trait",
+ "dr-types",
+ "log",
+ "thiserror 2.0.20",
+]
+
 [[package]]
 name = "dr-types"
 version = "0.1.0"
@@ -5158,6 +5168,15 @@ version = "1.1.2+spec-1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7d56353a2a665ad0f41a421187180aab746c8c325620617ad883a99a1cbe66d2"
 
+[[package]]
+name = "traceability"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "tracing"
 version = "0.1.44"