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.

core/dr-gpu/src/error.rs

@@ -1,3 +1,4 @@
+/// TRACES: NFR-R7 | NFR-R8
 /// Failures from the GPU layer.
 ///
 /// `DeviceLost` is deliberately a distinct variant rather than folded into a