Add image-trust signal to the verify verdict, backed by a stub allowlist (STE-54)

[yvesfracari]

Closes STE-54

What

Adds a second, orthogonal trust dimension to the verification verdict: every verify result now carries an imageTrust tier describing how trustworthy the declared build image is. Rationale (from the RFP): reproducibility alone is not faithfulness to source — a hostile build image can deterministically rewrite bytes and still pass byte-comparison, so image trust must be a graded signal, not a binary.

Tier	Meaning
sdf-trusted	image digest is on the SDF-trusted allowlist
publicly-auditable	allowlisted as a publicly-auditable third-party image
arbitrary	a bldimg was declared but is not allowlisted
unknown	no bldimg metadata available

How

• docker/allowlist.json (new): checked-in allowlist mapping image references/digests to a tier, seeded with this repo's pinned toolchain image from docker/toolchain-manifest.json (publicly-auditable, digest null while the image is only built locally) and a documented placeholder entry shape for future SDF stellar-cli-docker digests (sdf-trusted). Doc header records the eviction semantics: removing an entry downgrades the tier reported for past verifications; it never deletes verification records.
• reader/src/image-trust.ts (new): ImageTrustTier, AllowlistEntry, pure deriveImageTrust(bldimg, allowlist) (digest match or exact-reference match, first hit wins), and loadAllowlist() (missing file → empty allowlist; malformed file or an entry granting a non-allowlistable tier throws).
• reader/src/verify.ts: VerificationResult gains imageTrust + bldimg; trust is judged on the on-chain WASM's SEP-58 bldimg (exposed by STE-53). verifyById loads the default allowlist (overridable via options); the ERROR path reports unknown (no on-chain bytes → no metadata to judge).
• CLI: verify prints a trust: line with the tier and the bldimg value; --json includes the new fields automatically. Exit-code semantics unchanged (0 only on FULL_MATCH).
• Exported from the package index; README documents the tier table.

Testing

• 15 new unit tests cover all four tiers via deriveImageTrust, allowlist loading/validation, and compareWasm carrying the tier through every verdict (including: trust judged on on-chain metadata, not the rebuilt WASM's).
• Full reader suite passes: 57 passed, 3 skipped (opt-in live).
• Live testnet integration (SOROSCAN_INTEGRATION=1): fixture still reads/verifies; a new test derives its tier from on-chain metadata — unknown, correct since the fixture predates bldimg stamping.
• End-to-end CLI against the live fixture:

[OK]  FULL_MATCH
    contract:  CDVSGPL3HFBGJ6ZEYQUAVE3OH3XE2ZE5ZT2GWPA3LKOYVD4UBPQJ2VHB
    network:   testnet
    trust:     unknown (no bldimg metadata)
    on-chain:  6fe7bd58e5a33dc27daefc74acfae6eb70f101fdbde860475cf18fde87288e4b
    rebuilt:   6fe7bd58e5a33dc27daefc74acfae6eb70f101fdbde860475cf18fde87288e4b

[linear-code]

STE-54