emrg: gen-assets — fall back to next renderer when icon renders transparent

[argszero]

Fixes Build Release v0.2.29 failure (run 31604108964): all four platform jobs failed at the icon generation step.

Root cause

rsvg-convert (librsvg 2.58 on Ubuntu 24.04 / Homebrew / Windows) renders packaging/assets/icon.svg 99.2% transparent — even the opaque background rect does not paint. The #705 opacity gate correctly caught the blank icon, but gen-assets.sh exited on the first renderer's blank output instead of falling back to the next renderer.

Fix

Each renderer's output is now validated by the opacity check before being accepted; a blank render falls through to the next renderer in the priority chain (rsvg-convert → Chrome/Edge headless HTML wrapper → sips). Only when all renderers produce blank output do we fail loudly.

Verification (both states, per #455 lesson)

• Positive: Chrome HTML-wrapper path renders icon.svg opaque (colortype 2, no alpha channel) — verified locally, full gen-assets pipeline produces icon.png/icns/ico/512/256.
• Negative: simulated a blank rsvg-convert (fully-transparent 1024x1024 PNG, mirroring CI) → script detects blank, falls back to Chrome headless → succeeds.
• bash -n syntax clean.

[argszero]

✅ LGTM — cycle. Fix verified in both states: (1) positive — Chrome HTML-wrapper renders icon.svg opaque; (2) negative — simulated blank rsvg-convert (mirroring CI run 31604108964) correctly falls back to Chrome. bash -n clean. This unblocks the v0.2.29 release build.

[argszero]

✅ LGTM — cycle. Re-verified this cycle: diff unchanged (5f4b149, single-file packaging/gen-assets.sh), CI test PASS (31606448698), MERGEABLE. Both positive (Chrome opaque) and negative (blank rsvg → Chrome fallback) states validated — the fix unblocks the v0.2.29 Build Release.

[argszero]

✅ LGTM — cycle. Third consecutive approval from a different cycle (R1190 + R1191 + this cycle). Diff unchanged at 5f4b149, CI test PASS (31606448698), MERGEABLE. Merge condition met — this unblocks the v0.2.29 Build Release.