View tag matching

[clintjeff2]

Motivation

• Ensure the Stellar scan migration boundary between the new public-announcement view-tag and the legacy shared-secret view-tag is tested so optimized scanning doesn't silently break compatibility.
• Provide regression coverage that the optimized scanner rejects legacy-tagged announcements while the legacy scanner (scanAnnouncementsLegacySharedSecretTag) still finds them for migration and benchmarking.

Description

• Updated test/chains/stellar/scan.test.ts to import computeSharedSecret, computeViewTag, and scanAnnouncementsLegacySharedSecretTag from the Stellar codepath.
• Added a new test keeps legacy shared-secret view tags on the legacy scanner path that builds a deterministic legacy-tagged announcement, asserts the optimized scanAnnouncements rejects it, and asserts the legacy scanner finds it.
• The change exercises both tag derivations and preserves existing benchmark/compatibility tooling without altering runtime scan logic.

Testing

• Ran typecheck with pnpm exec tsc --noEmit, which succeeded without errors.
• Ran the targeted test with pnpm test -- test/chains/stellar/scan.test.ts, which passed (all tests in that file succeeded).
• Ran the full test suite with pnpm test, which completed successfully with all tests passing (136 passed).
• Ran the benchmark STELLAR_SCAN_BENCH_SIZES=10000 pnpm exec vitest bench test/chains/stellar/bench/scan.bench.ts --run, which reported an approximately 229× speedup for the optimized public-prefilter scan over the legacy shared-secret-tag scan on the 10k synthetic dataset.

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Closes #4

[truthixify]

229× speedup on the 10k dataset is well past what #4 asked for, and keeping the legacy scanAnnouncementsLegacySharedSecretTag path alongside the optimized one with a migration-boundary regression test is the right call for safe rollout. Design doc covers the privacy tradeoff explicitly. Merging. Thanks @clintjeff2.