feat(report): host-declared surface meta in status reports

[ranxianglei]

Closes #278.

What

StatusReportOptions gains optional meta: { pack?, packVersion?, host? }. The overview and the compressed drilldown render a leading:

ACTIVE SURFACE: pack=lean v3 | host=billion-context-pi 0.1.69

No meta → reports stay byte-identical.

Design

• SurfaceMeta interface + renderSurfaceLine() in src/report.ts; SurfaceMeta re-exported from the barrel (src/index.ts).
• Which pack is active is host policy — the kernel never resolves packs, it only renders what the host declares (billion-context #730 division of labor).
• Line is prefixed with a blank-line separator to the overview and compressed-drilldown outputs only. Uncompressed ranges/messages views are untouched, per issue scope.
• Empty/undefined fields are filtered: partial meta renders only the parts present (pack=v3 when only packVersion given); all-empty meta ({}) behaves exactly like absent meta.

Verification

• tsc --noEmit clean; full suite 731/731 pass (7 new tests in tests/surface-meta.test.ts: no-meta byte-clean, empty-meta equivalence, overview leading line, compressed leading line, partial-meta variants ×3, uncompressed views never get the line).
• Byte-identity against pre-change code: ran HEAD's buildStatusReport vs new one over a fixture state (nested T1/T2 blocks + inactive block + tool messages) across all scope/view/sort/limit combos — all 8 outputs byte-identical without meta.
• npm run build succeeds.

[github-actions]

📦 Built Package Artifact

Branch: 2026-09-13_surface-meta-status (342efe9)

Option A — Install from npm PR tag (recommended)

In your adapter project:

npm install acp-kernel@pr-280

Each push to this PR publishes a new version under the pr-280 npm tag.

Option B — Download artifact

1. Download the artifact from the Actions run
2. Extract the tarball and install:

tar xzf acp-kernel-pr280.tgz
npm install ./package

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