Priority 2 Track D: reduce frontend build noise

[rendall]

Summary

• formalize and execute Priority 2 Track D for frontend build-noise hygiene
• remove the repo-local stylesheet-path build warning while preserving the emitted /css/simple-comment-style.css contract
• document the two remaining frontend build messages as intentional residual notices with owner and re-check triggers
• include the small docs/norms/implementation.md start-state wording refinement already present on this branch

Scope guard

This PR does not:

• upgrade Vite, Svelte, or carbon-icons-svelte
• expand into frontend feature or UI work
• replace dependencies just to reduce warning count
• silently suppress build messages without documentation

Frontend build outcome

Before Track D:

• The CJS build of Vite's Node API is deprecated.
• [vite-plugin-svelte] WARNING: ... no exports condition for svelte
• /css/simple-comment-style.css doesn't exist at build time, it will remain unchanged to be resolved at runtime

After Track D:

• the stylesheet-path warning is removed via a placeholder-based HTML/Vite transform
• the emitted built HTML still points to /css/simple-comment-style.css
• the remaining Vite CJS notice and carbon-icons-svelte warning are documented as tolerated residual notices

Validation

• yarn run build:frontend
• bash ./scripts/validate-frontend-artifacts.sh dist
• bash ./scripts/smoke-frontend-embed.sh dist
• yarn run ci:local

Validation notes

• yarn run ci:local passed end to end.
• During the initial yarn --frozen-lockfile --production=false step, Yarn reported an optional unix-dgram native build failure under Node 22, but marked it safe to ignore and continued successfully.

Evidence

• docs/plans/Priority2TrackDFrontendBuildNoiseHygienePlan.md
• docs/plans/Priority2TrackDFrontendBuildNoiseHygieneChecklist.md
• docs/archive/priority2-track-d/build-frontend-baseline.log
• docs/archive/priority2-track-d/build-frontend-after.log
• docs/archive/priority2-track-d/warning-before-after.md
• docs/archive/priority2-track-d/warning-register-current.md
• docs/archive/priority2-track-d/validation-report.md
• docs/archive/priority2-track-d/residual-notice-note.md