Add stable machine-parseable receipt schema to top-level CLI (publish / fetch-metrics)

[AlyciaBHZ]

Why

broadcast-kit is increasingly consumed as the egress executor behind other systems (the FKST reddit package already shells out to it; more FKST marketing packages — social-metrics, douyin/xhs publishers — will follow). Today those consumers must scrape inconsistent JSON: URL fields differ per platform (post_url / note_url / video_url), status values vary (success / ok / failed), and per-publisher CLIs print human text. Every consumer ends up writing brittle parsing.

This PR gives the top-level broadcast-kit publish and broadcast-kit fetch-metrics a stable, versioned receipt contract so any caller can depend on fixed field names/types instead of reverse-engineering output.

What

• New broadcast_kit/receipts.py normalizes only the top-level command result just before print.
• Two receipt types:
  • broadcast.publish.receipt.v0 — always-present: platform, status (ok|dry_run|blocked|error), account_label, dry_run, post_uri (unified from post_url/note_url/video_url/url), evidence_path, reason.
  • broadcast.metrics.fetch_receipt.v0 — always-present: platform, status (ok|dry_run|stub|error), account_label, dry_run, metrics_path (incl. legacy source.raw_path fallback), records, preview, reason.
• Backward compatible: legacy fields preserved; original status kept as raw_status when remapped; for fetch-metrics --platform all the nested records items are left untouched (top-level-only normalization).
• XHS metrics stays honest status: "stub" (no faked collector).
• Unknown fetch statuses normalize to error (safe default for a contract).
• Contract doc: contracts/receipts.md — including the rule that automation should parse only the top-level publish / fetch-metrics JSON, never the per-publisher CLIs (which are non-contract).
• Tests: tests/test_cli_receipts.py — 28 tests pass, covering key presence, records preservation for --platform all, raw_status retention, URL/evidence normalization, and the source.raw_path fallback.

Non-goals

No changes to publisher/collector behavior, per-publisher CLIs, or any live-publish path. Purely additive output normalization.

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