feat(signature-hint-auditor): map envelope signature hints to public candidates

[chainsmt]

Closes #467

Summary

Adds an offline Transaction Signature Hint Auditor under features/signature-hint-auditor/. Paste a transaction envelope XDR, optionally provide public G… signer candidates, and see every decorated signature’s four-byte hint mapped to zero/one/many candidates — without treating a hint match as cryptographic verification.

Motivation

Envelope XDR already carries decorated signature hints, but operators often confuse “hint matches this pubkey” with “signature verified.” Four bytes collide. Fee-bump envelopes also mix outer fee-source signatures with inner transaction signatures. This tool separates groups and labels matches honestly.

What this PR does

• Decodes supported classic (v0/v1) and fee-bump envelopes locally with @stellar/stellar-sdk — no Horizon / no network (msw/handlers.ts is empty).
• Lists every decorated signature hint in original order (lib/decorated-signatures.ts).
• Accepts optional public signer keys and derives their hints the network way (last four bytes of the raw public key) via lib/signer-hints.ts.
• Maps hints to zero, one, or many candidates and surfaces collisions (lib/hint-candidates.ts + CollisionNotice).
• Keeps fee-bump outer and inner signature groups separate in the UI (SignatureRows).
• Labels every match as a hint match, never a verified signature.
• Refuses secret seeds (S…) on prefix alone in both envelope and signer fields and remounts the form so seeds do not linger in the textarea. Input stays memory-only.
• Error codes with dedicated copy: empty_xdr, invalid_xdr, invalid_public_signer, unsupported_envelope, too_many_signers.
• Full feature contract plus deterministic fixtures for signatures and forced hint collisions; e2e covers the ambiguous-hint path.

Architecture notes

• Slice-only: features/signature-hint-auditor/.
• Collision fixture forces two genuine public keys to share a recorded hint so the ambiguous UI is deterministic.
• Related reading: patterns similar to fee-bump-inspector hint listing, but this slice is independent.

Acceptance criteria coverage

• Decode supported envelopes; list every decorated hint in original order
• Optional public signers; derive hints; never accept secret seeds
• Zero/one/many candidates per hint; identify collisions
• Separate fee-bump outer vs inner groups
• Label matches as hint matches only; keep input in memory

Out of scope

• Cryptographic signature verification
• Threshold / weight checks
• Transaction submission
• Network lookups

Test plan

• npm run verify:features -- signature-hint-auditor (35 files)
• npm test -- features/signature-hint-auditor (73 tests)
• Manual: signed classic envelope + matching pubkey → single hint match
• Manual: fee-bump envelope → outer and inner groups
• Manual: paste S… seed → refused and cleared
• Manual: collision fixture → multiple candidates called out

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