feat: migrate to visill 0.1.0-rc

[jordanl17]

Long-lived migration PR for the visill framework rollout. Tracks the floating @rc dist-tag through the rollout window and merges to main only at visill stable cut (0.1.0 on @latest) per ADR 0020.

What changed

• vite.config.ts drops the inline finalizeBundle function and adopts import { finalizeBundle } from '@visill/build' with explicit { outDir: assetsDir }. TF retains manual plugin composition (viteSingleFile + finalizeBundle) rather than defineVisillConfig({}) because TF's skill artefacts live under targetable-feedback/ directly (no skill-src/ source, no assembleSkill flow).
• widget-src/widget.ts replaces the inline requireElement and ownDescendant helpers with @visill/sdk imports. All ownDescendant call sites now pass '.unit' as the boundary selector (third arg, added in @visill/sdk@0.1.0-rc.2 to support TF's nested .unit > .guidance-wrap > .target pattern). Negated guards rewritten to positive form per the visill conventions.
• Local dev: pnpm.overrides link the three @visill/* packages to sibling source paths so SDK edits propagate without republish.
• CI: .npmrc.ci + jq 'del(.pnpm)' strips the overrides block so CI installs the published RC.
• scripts/parity-check.sh implements the five parity gates from PRD 007 §6. Gate 1 was extended to treat render.py as optional, since TF has no Mustache-rendered skill artefacts. Wired into .github/workflows/build.yml.
• Node 24: engines.node bumped to >=24.0.0, @types/node to ^24.0.0, workflow node-version to '24', matching the visill workspace baseline.

Parity gates (local smoke vs prior main)

Gate	Status	Notes
1. zip file-list + render.py mode bits	PASS	file lists identical, no render.py in either zip
2. per-file byte-equal except widget-bundled.html	PASS	only widget-bundled.html differs
3. structural HTML diff (data-island + style)	PASS	module-script body excluded per ADR 0021 - migration rewires helpers by design
4. gzipped bundle size delta ±5%	PASS	delta 3.04%
5. eval grading re-run	SKIPPED	no checked-in transcripts; deferred per ADR 0021

Do not merge yet

This PR is intentionally open through Phase 10. At stable cut: bump "rc" pins to ^0.1.0, drop pnpm.overrides, and merge.