feat: add swatch-grid example with smoke-gate coverage

[TMHSDigital]

Stage 1 of 3 — promote the functional swatch-grid example, smoke-gated

Adds examples/swatch-grid/ (a runnable, dependency-light demo) and wires the shipped file into the blender-smoke gate on both matrix builds.

Decisions applied (defaults from the task; not contentious)

• Model: examples/ is smoke-gated content but not added to validate-counts. The README aggregate (12 skills, 6 rules, 2 templates, 17 snippets) is unchanged.
• Version: a feat: cuts 0.3.0 under the gate. Renumbered the ROADMAP version-theme table (editable rows only, not the **Current:** line): v0.3.0 = Examples and demos (smoke-gated), and moved the 5.2 LTS sweep → v0.4.0. Candidate-pool heading renumbered to match.

What it teaches / proves

• Procedural-materials patterns end to end: Principled metal (gold/copper) + dielectric (red/blue plastic, white rough), the emission pattern, and the set_specular shim (Specular → Specular IOR Level), via string socket lookups and 4-tuple colors.
• Doubles as the EEVEE engine-id proof: asserts BLENDER_EEVEE on 5.x / BLENDER_EEVEE_NEXT on 4.2–4.5 before rendering.

Smoke coverage (runs the real example file)

blender-smoke.yml adds a step that runs examples/swatch-grid/swatch_grid.py --engine cycles --samples 8 --width 640 on each build. The example self-verifies non-black AND distinct-region-count == material-count and exits non-zero on failure. Cycles (CPU) for pixels on the GPU-less runner; the EEVEE id is still asserted (frame-independent).

Verified locally (Blender 4.5.10 LTS + 5.1.1)

• CI-mode (Cycles): both builds → id OK, set_specular=Specular IOR Level, 6 distinct regions, exit 0.
• Default EEVEE: 5.1.1 → BLENDER_EEVEE, 6 regions, exit 0.

No screenshot here (Stage 3). No HDRI/network dependency (Stage 2 is the hero variant).

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