Missing per-contract READMEs (announcer, batch-sender, vault, asset-policy, metrics, governance)

[truthixify]

Tier: S (1-2 days) | Type: docs

Context. Only 4 of 10 Stellar crates ship a README: stealth-registry/README.md, stealth-sender/README.md, stealth-splitter/README.md, wraith-names/README.md. The other six (stealth-announcer, stealth-batch-sender, stealth-vault, wraith-asset-policy, wraith-metrics, contracts/governance) have no README, so contributors have to read the source to learn the API surface, error variants, event topics, and pause / admin / metrics posture. This becomes load-bearing for the third-party audit engagement (issue #4).

Scope.

• Add a README.md to each of the six crates using a shared template: purpose, entrypoints table, error variants, event topics, storage layout, pause / admin / metrics posture, related docs.
• Cross-link each new README to stellar/PAUSE.md, stellar/MULTISIG.md, stellar/METRICS.md, and stellar/ONCALL.md from issue Cross-chain announcement schema conformance audit #12.
• Add a single line to stellar/README.md's contracts table linking each new README.
• Where a crate is deliberately frozen (announcer) or a library (metrics) or explicitly NOT PRODUCTION (governance PoC), say so in bold at the top.

Acceptance.

• Every Stellar crate under stellar/contracts/* and stellar/* (except 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 one

Files. stellar/stealth-announcer/README.md (new), stellar/stealth-batch-sender/README.md (new), stellar/stealth-vault/README.md (new), stellar/wraith-asset-policy/README.md (new), stellar/wraith-metrics/README.md (new), stellar/contracts/governance/README.md (new), stellar/README.md.