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.

docs/traceability.md

@@ -0,0 +1,187 @@
+# Requirements traceability matrix
+
+<!-- GENERATED FILE — do not edit by hand. -->
+<!-- Regenerate: cargo run -p traceability -- report -->
+
+Denominators are parsed from [`requirements.md`](requirements.md) at run time, never hardcoded. Coverage is the intersection of tagged and defined IDs over defined IDs, so it cannot exceed 100%.
+
+## Summary
+
+| Metric | Value |
+|---|---|
+| Source files scanned | 16 |
+| TRACES tags found | 16 |
+| Requirements defined | 143 |
+| Requirements covered | 19 |
+| **Coverage** | **13.3%** (19/143) |
+
+### By type
+
+| Type | Covered | Defined |
+|---|---|---|
+| FR | 13 | 90 |
+| NFR | 4 | 47 |
+| R | 2 | 6 |
+
+## Orphan tags
+
+A tag naming an ID `requirements.md` does not define — what renumbering produces, and what a typo produces.
+
+_None._
+
+## Tagged requirements
+
+| ID | Tagged in |
+|---|---|
+| FR-CAT-1 | [`tools/traceability/src/lib.rs:473`](../tools/traceability/src/lib.rs#L473), [`tools/traceability/src/lib.rs:505`](../tools/traceability/src/lib.rs#L505) |
+| FR-CAT-1a | [`core/dr-types/src/lib.rs:23`](../core/dr-types/src/lib.rs#L23) |
+| FR-CAT-2 | [`tools/traceability/src/lib.rs:473`](../tools/traceability/src/lib.rs#L473) |
+| FR-DEV-4 | [`core/dr-gpu/src/lib.rs:119`](../core/dr-gpu/src/lib.rs#L119) |
+| FR-DSP-1 | [`ui/dr-ui/src/lib.rs:167`](../ui/dr-ui/src/lib.rs#L167) |
+| FR-NC-12 | [`core/dr-sync/src/lib.rs:127`](../core/dr-sync/src/lib.rs#L127), [`core/dr-sync/src/lib.rs:33`](../core/dr-sync/src/lib.rs#L33) |
+| FR-NC-3 | [`core/dr-sync/src/capability.rs:41`](../core/dr-sync/src/capability.rs#L41) |
+| FR-NC-4 | [`core/dr-sync/src/capability.rs:6`](../core/dr-sync/src/capability.rs#L6), [`core/dr-sync/src/lib.rs:127`](../core/dr-sync/src/lib.rs#L127) |
+| FR-NC-6c | [`core/dr-types/src/lib.rs:111`](../core/dr-types/src/lib.rs#L111) |
+| FR-PLAT-AND-1 | [`core/dr-types/src/lib.rs:23`](../core/dr-types/src/lib.rs#L23) |
+| FR-RAW-1 | [`core/dr-types/src/lib.rs:70`](../core/dr-types/src/lib.rs#L70) |
+| FR-UI-1 | [`ui/dr-ui/src/lib.rs:188`](../ui/dr-ui/src/lib.rs#L188) |
+| FR-UI-2 | [`ui/dr-ui/src/lib.rs:188`](../ui/dr-ui/src/lib.rs#L188) |
+| NFR-OPS-1 | [`tools/traceability/src/lib.rs:266`](../tools/traceability/src/lib.rs#L266) |
+| NFR-P1 | [`tools/traceability/src/lib.rs:473`](../tools/traceability/src/lib.rs#L473) |
+| NFR-R7 | [`core/dr-gpu/src/error.rs:1`](../core/dr-gpu/src/error.rs#L1) |
+| NFR-R8 | [`core/dr-gpu/src/error.rs:1`](../core/dr-gpu/src/error.rs#L1) |
+| R1 | [`tools/traceability/src/lib.rs:489`](../tools/traceability/src/lib.rs#L489), [`tools/traceability/src/lib.rs:493`](../tools/traceability/src/lib.rs#L493) |
+| R4 | [`core/dr-gpu/src/lib.rs:119`](../core/dr-gpu/src/lib.rs#L119) |
+
+## Not yet tagged
+
+124 of 143 requirements have no implementation tag. Expected while the codebase is young; each should gain one as it is built.
+
+<details><summary>Show untagged requirements</summary>
+
+- FR-CAT-10
+- FR-CAT-11
+- FR-CAT-12
+- FR-CAT-13
+- FR-CAT-14
+- FR-CAT-3
+- FR-CAT-4
+- FR-CAT-5
+- FR-CAT-6
+- FR-CAT-7
+- FR-CAT-8
+- FR-CAT-9
+- FR-CULL-1
+- FR-CULL-2
+- FR-CULL-3
+- FR-CULL-4
+- FR-CULL-5
+- FR-CULL-6
+- FR-CULL-7
+- FR-DEV-1
+- FR-DEV-2
+- FR-DEV-3
+- FR-DEV-3a
+- FR-DEV-3b
+- FR-DEV-3c
+- FR-DEV-3d
+- FR-DEV-3e
+- FR-DEV-3f
+- FR-DEV-3g
+- FR-DEV-5
+- FR-DEV-6
+- FR-DEV-7
+- FR-DEV-8
+- FR-DSP-2
+- FR-DSP-3
+- FR-DSP-4
+- FR-DSP-5
+- FR-DSP-6
+- FR-DSP-7
+- FR-DSP-8
+- FR-EXP-1
+- FR-EXP-2
+- FR-EXP-3
+- FR-EXP-4
+- FR-EXP-5
+- FR-EXP-6
+- FR-EXP-7
+- FR-EXP-8
+- FR-EXP-9
+- FR-NC-1
+- FR-NC-10
+- FR-NC-11
+- FR-NC-2
+- FR-NC-5
+- FR-NC-6
+- FR-NC-6a
+- FR-NC-6b
+- FR-NC-7
+- FR-NC-8
+- FR-NC-9
+- FR-PLAT-AND-2
+- FR-PLAT-AND-3
+- FR-PLAT-AND-4
+- FR-PLAT-AND-5
+- FR-PLAT-AND-6
+- FR-PLAT-LIN-1
+- FR-PLAT-LIN-2
+- FR-PLAT-LIN-3
+- FR-RAW-2
+- FR-RAW-3
+- FR-RAW-4
+- FR-RAW-5
+- FR-UI-3
+- FR-UI-4
+- FR-UI-5
+- FR-UI-6
+- FR-UI-7
+- NFR-A11Y-1
+- NFR-A11Y-2
+- NFR-A11Y-3
+- NFR-ARCH-1
+- NFR-ARCH-2
+- NFR-ARCH-3
+- NFR-ARCH-4
+- NFR-COMPAT-1
+- NFR-COMPAT-2
+- NFR-OPS-2
+- NFR-OPS-3
+- NFR-OPS-4
+- NFR-P10
+- NFR-P11
+- NFR-P12
+- NFR-P13
+- NFR-P14
+- NFR-P15
+- NFR-P2
+- NFR-P3
+- NFR-P4
+- NFR-P5
+- NFR-P6
+- NFR-P7
+- NFR-P8
+- NFR-P9
+- NFR-PORT-1
+- NFR-PORT-2
+- NFR-PORT-3
+- NFR-R1
+- NFR-R2
+- NFR-R3
+- NFR-R4
+- NFR-R5
+- NFR-R6
+- NFR-RES-1
+- NFR-RES-2
+- NFR-RES-3
+- NFR-RES-4
+- NFR-SEC-1
+- NFR-SEC-2
+- NFR-SEC-3
+- NFR-SEC-4
+- R2
+- R3
+- R5
+- R6
+
+</details>