docs: add per-contract READMEs for six Stellar crates (#167)

[boalambo]

Summary

Adds README.md files to the six Stellar crates that were missing documentation, closing #167. This documentation is load-bearing for the upcoming third-party audit engagement (#4).

Changes

• stealth-announcer/README.md — Frozen, stateless event emitter (v2 announcement schema). Marked ⚠️ DELIBERATELY FROZEN at the top.
• stealth-batch-sender/README.md — Batch stealth transfers, ~100x efficiency gain over individual sends.
• stealth-vault/README.md — Time-locked vault with refund safety net.
• wraith-asset-policy/README.md — Admin-controlled asset allowlist.
• wraith-metrics/README.md — Shared metrics library. Marked ⚠️ LIBRARY, NOT A DEPLOYABLE CONTRACT.
• contracts/governance/README.md — Token-weighted governance PoC. Marked ⚠️ NOT PRODUCTION READY — PROOF OF CONCEPT ONLY.
• stellar/README.md — Updated contracts table to link all 10 crate READMEs.

Each new README follows the shared template:

• Purpose / overview
• Entrypoints table (with authorization)
• Error variants (as they appear in source)
• Event topics with data structures
• Storage layout (incl. TTL strategy where relevant)
• Pause / admin / metrics posture
• Cross-links to PAUSE.md, MULTISIG.md, METRICS.md, and ONCALL.md

Notes

• ONCALL.md is referenced per issue Cross-chain announcement schema conformance audit #12 but does not currently exist in stellar/. Links are left in place per maintainer instruction, for when that file is added.

Acceptance criteria

• Every Stellar crate under stellar/contracts/* and stellar/* (excluding bench, bench-crossover, integration-tests, shared) has a README
• Each README documents the full entrypoint list, error variants, and event topics as they appear in source
• stellar/README.md contracts table links to every crate README

Closes #167

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[truthixify]

Merged. Six READMEs in one pass and the entrypoint tables match source. Thanks @boalambo.