fix(parser): preserve content around opener/closer with trailing whitespace

[teezeit]

Summary

Fixes 4 bugs in remark-containers.ts (2 targeted + 2 incidental) and recovers content in 2 docs fixtures that was previously silently dropped.

Targeted bugs:

• ::: card (trailing spaces on opener) — remark emits a hard `break` node, splitting the opener paragraph into multi-child. `collectContainer` only handled `children.length === 1`, so post-opener content was silently dropped → empty container.
• ::: (trailing space on closer) — CASE 1 recognized the closer line via `.trim()` but silently discarded any text after the closer.

Cascade — incidentally fixed:

• `docs/page-layouts/sidebar-layout` and `sidebar-with-sections` had `.expected-fail.invariants.ts` for the same root cause (multi-child opener paragraph). Both invariants flipped to live.

Content recovery (no prior expected-fail invariants):

• `docs/tabs/basic-tabs` — `Free — basic plan` / `Pro — $12/mo` lines no longer dropped.
• `docs/navigation/sidebar-nav` — `App` brand line no longer dropped.

What changed

packages/core/src/parser/remark-containers.ts

1. `collectContainer` now returns `{ node, trailing?, nextIndex }`; `processNodes` and the two recursive callsites push trailing siblings after the container.
2. CASE 3 has a new `else if` branch for multi-child openers — extracts the post-opener children (skipping a leading hard `break` if present) into a synthetic content paragraph.

Tests

• New `tests/parser/remark-containers-whitespace.test.ts` — 3 isolated parse tests pinning the opener/closer whitespace contract (uses the parser-contract test helpers from PR refactor(parser): TransformContext with explicit lookahead #82).
• 4 fixture invariants flipped from `.expected-fail` to live; `KNOWN_FAILURES` in `review-gate.test.ts` trimmed.
• 6 `REVIEW_LOG.md` rows ⏳→✅ (HTML output verified by diff inspection — see note below).

Known caveat

The React/Tailwind snapshots for `basic-tabs` and `sidebar-nav` reveal a pre-existing latent bug: `transformParagraph` builds inline `strong`/`em` as HTML string literals (lines 776-786 of transformer.ts), which render correctly in HTML but escape to `<strong>` in React/JSX. This pattern already exists in 10+ other fixture snapshots — not introduced here. Separate PR will land a proper inline-children fix.

Test plan

• `pnpm typecheck` — clean
• `pnpm test` — 1,187 passed (1,184 baseline + 3 new isolated remark-containers tests)
• `pnpm review:refresh` — 6 fixtures flipped to ⏳ then verified ✅ (HTML output is correct; React/Tailwind escape is a separate latent bug)
• All 4 expected-fail → live invariants pass; no other expected-fail tests changed

🤖 Generated with Claude Code