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

core/dr-gpu/src/lib.rs

@@ -116,6 +116,7 @@ struct Params {
 /// Stands in for the develop pipeline in v0.1. What matters is the shape:
 /// compute writes a texture, the texture is handed to the compositor, and
 /// pixels never travel back through the CPU.
+/// TRACES: FR-DEV-4 | R4
 pub struct RenderTarget {
     ctx: GpuContext,
     texture: wgpu::Texture,
@@ -142,9 +143,7 @@ impl RenderTarget {
             .device
             .create_shader_module(wgpu::ShaderModuleDescriptor {
                 label: Some("gradient"),
-                source: wgpu::ShaderSource::Wgsl(
-                    include_str!("shaders/gradient.wgsl").into(),
-                ),
+                source: wgpu::ShaderSource::Wgsl(include_str!("shaders/gradient.wgsl").into()),
             });
 
         let bind_group_layout =
@@ -282,12 +281,8 @@ impl RenderTarget {
             return;
         }
         let (texture, view) = Self::create_texture(&self.ctx, width, height);
-        self.bind_group = Self::create_bind_group(
-            &self.ctx,
-            &self.bind_group_layout,
-            &view,
-            &self.params_buf,
-        );
+        self.bind_group =
+            Self::create_bind_group(&self.ctx, &self.bind_group_layout, &view, &self.params_buf);
         self.texture = texture;
         self.view = view;
         self.width = width;
@@ -369,10 +364,7 @@ impl RenderTarget {
         }
         let buf = &slot.as_ref().unwrap().0;
 
-        let mut enc = self
-            .ctx
-            .device
-            .create_command_encoder(&Default::default());
+        let mut enc = self.ctx.device.create_command_encoder(&Default::default());
         enc.copy_texture_to_buffer(
             wgpu::ImageCopyTexture {
                 texture: &self.texture,