fix(webkit): state Required as true/false across every spec props table

[gabriel-lisboa-azion]

Summary

Follow-up to @robsongajunior's review on #873: the Required column of a spec's Props table read yes / no while the Type and Default columns beside it read as literal values, so one table spelled the same boolean fact two ways. Every spec now says true / false, and .specs/_template.md teaches it (plus the escaping rule below) so the next spec does not regress.

Four things fall out of the sweep:

• Seven specs carried unescaped pipes inside their union types — button, card-pricing, currency, icon-button, message, status-indicator, tag. A bare | closes the cell, so those rows render with extra columns on GitHub and their Required cell sat in the wrong column. The pipes are now escaped ('a' \| 'b'), which is also what made those cells reachable by the sweep.
• Every checksum is recomputed. enforce-spec-exists hashes the spec body, so editing a table without re-hashing would block the next /component-create on that component.
• catalog.json is rebuilt. build-catalog.mjs copies the Required cell verbatim, so the column change is catalog drift until it is regenerated.
• The MCP usage-snippet generator needed a fix (src/mcp/queries.js): exampleProps matched the literal 'yes', so with true in the tables it stopped seeing any prop as required and fell back to leading scalars — box-grid-selection's generated snippet became modelValue="undefined" instead of :items="[]". It now accepts both spellings, so it reads correctly whichever specs have migrated. This is the one code change here; test/mcp/queries.test.mjs caught it, which is why the same fix also rides in feat(webkit): add the Hint component, Label's hint prop, and quieter field-* helpers #873.

.specs/_template.md is a shared doc, so per .claude/rules/git-workflow.md it ships here rather than riding along with #873. #873 already carries the column change for the 8 specs it touches (hint, label, field-text, field-select, field-password, field-text-switch, field-phone-number, field-input-group), so this PR deliberately leaves those files alone — no conflict either way.

No .vue, no story, no component behavior change. scroll-area, spinner and tab-view are untouched: their Props tables are _none_.

How to test

1. grep -rE '\| *(yes|no) *\|' .specs/*.md → no hits.
2. node -e over .specs/*.md with parseSpecFile + bodyChecksum from .claude/hooks/_lib/spec.mjs → every frontmatter checksum matches its body (verified: 0 mismatches across 82 specs).
3. cd packages/webkit && node --test "test/mcp/*.test.mjs" → 28/28.
4. node .claude/hooks/__tests__/run.mjs → 32/34, the same two pre-existing failures as main (legacy-whitelist + enforce-spec-exists fixture; both fail identically on a clean main checkout).
5. Open .specs/button.md on GitHub → the Props table now renders 5 columns on every row.

Notes

• Formatting is left exactly as each file had it. 75 specs already fail prettier --check on main and .specs/ is not covered by any format gate (webkit:format:check runs inside packages/webkit), so reformatting them would have buried this diff.
• The catalog stores "required": "false" as a string, because build-catalog.mjs copies the cell text. Making it a real JSON boolean would read better but changes the catalog's shape for anything consuming it, so it is left alone here.