fix(platform): stage accepted task attachment metadata

[yannickmonney]

What changed

TALE-617, bounded child B of #4101 / TALE-484. The parent remains open for
other follow-ups and rollout; this PR does not close #4101.

• Accept the attachment door's existing 1–255 UTF-16 name units and 0–255 MIME
  units in the review manifest, retaining the original display metadata.
• Separate the on-disk leaf: reuse safePathSegment and stop at 240 UTF-8 bytes
  on code-point boundaries. Existing hashed storage-ref directories isolate
  distinct blobs even when normalized leaves share a prefix.
• Do not infer a MIME type from names. The signed blob stream already uses the
  object's type with application/octet-stream fallback where required; task
  display MIME is not forwarded into transfer headers.
• Keep controls, membership/tenant/document ACL gates, exact signed sizes,
  the 20 MiB limit, live issuer/current grant, and pre/post-transfer checks.
• Extend existing unit/manifest tests and the PG/MinIO/actual-daemon lane using
  production attachment/output service writers. No second transfer stack or
  daemon, TTL, retention, durable audit, grant or opt-in change.

Verification

• Exact origin/main implementation with the final causal matrix: 15 expected
  failures / 35 passes (blank MIME, 255-unit MIME, 241/255 ASCII and >240-byte
  multibyte names, plus shared-prefix isolation).
• Changed unit/manifest suite and adjacent review, signed-token, signed-stream,
  discovery and integration-skip guards: 146 tests pass across 6 files with
  installed Node 24.21.0, one worker, unchanged Vitest config.
• Scoped oxlint/oxfmt and git diff --check pass. Selected-root TypeScript checks
  cover the four changed files and both direct shim importers, with imports
  resolved: zero diagnostics; not whole-workspace tsc.
• Scoped pinned Opengrep: zero findings on four files; engine reports one
  excluded item, so this is not a claim of exhaustive analysis.

Open proof / environment limitations

• No heavy stack started. Named H1/H2 request is pending with fleet manager
  (30 minutes, estimated additional peak disk 8 GiB, 20 GiB reserve plus growth).
  New real-byte integration cases are implemented, not yet executed.
• Existing dependent bridge, attachment/output service and shim tests cannot
  collect because the installed tree lacks eventsource-parser. Raw logs are
  retained; no reinstall, cache wipe or configuration weakening attempted.
• Hosted checks and distinct independent review must finish before acceptance.
  TALE-618 follows under the same shared-test owner; TALE-549 remains separate.

Current-main rebase

Replayed the previously accepted source 5a88a660669810906eff598585071ce0c0990780 onto main d1373d84cd56972501403f62145ec52e6f65d44a, including the merged shared CI repair in #4625. The accepted feature payload and all current-main changes are preserved in one atomic commit. Configured commit and conflict checks pass; earlier behavioral proof remains recorded above. All seven native required checks and full merge-group validation remain required for this new source.

Maintenance replay: preserves the accepted feature payload on current main 7d178ca. Includes the merged #4649 Knip cleanup and the exact independently accepted one-line shared CLI inventory repair from #4654 (252f0df), which is still pending native merge on main. The fixed suite inventory keeps its discovery and source/compiled phase guards. Existing feature proof is retained; no fresh full-feature/full-workspace test or hosted-green claim. Native required checks remain mandatory.

[yannickmonney]

TALE-635 independent review — ACCEPT at source level only

Exact head: 12598c9d5af9d03d34cbd2407454a90fb546e3e7 (base ffa15e019c3b6453bfb82dc44231abef2b8be0de), reverified before posting. Reviewer Codex #1 af98a247, run d969b4ff, is distinct from implementation Codex #6 8c78cac9, run 2d54b5a0. Reviewed the full TALE-617 contract and four-file patch adversarially. No blocking source defect found.

Evidence:

• The original display name/MIME, file ID, kind and output provenance remain intact. reviewFileLeaf reuses safePathSegment and bounds the disk leaf to 240 UTF-8 bytes on code-point boundaries. Separator/dot normalization prevents traversal; SHA-256 storage-reference directories separate distinct blobs even with identical normalized prefixes.
• Blank/long display MIME is not a transfer header and is not inferred from the filename. The inherited signed object stream uses the object's type, with application/octet-stream when absent. No invented MIME.
• Controls/DEL, ambiguous membership, foreign metadata, document-bound aliases, exact sizes and 20 MiB ceiling remain refused. Live issuer/current grant, org/project/captured-source gates and pre/post-transfer checks remain unchanged. Late revocation/handoff refuses a successful current receipt; historically snapshot-authorized bytes may remain, not be erased.

Independent light reproduction (installed Node v24.21.0, existing server config, one worker):

• Exact head: 146 PASS across six files, exit 0.
• Exact original production implementation with head regression tests: 15 expected FAIL / 35 PASS, exit 1. Blank/255-unit MIME, 241/255-unit ASCII, >240-byte multibyte and shared-prefix regressions reproduce independently.
• Five dependent bridge/attachment/output/direct-shim suites: 59 PASS, exit 0.
• Four-file oxlint and oxfmt, plus diff whitespace: PASS.

Environment correction: the author's missing eventsource-parser limitation did not reproduce in this standing clone: existing installed version is 4.1.1, and all five selected dependent suites actually collected. No package was installed by this reviewer; this does not reinterpret the author's original environment/logs.

OPEN MERGE GATE: the extended isolated PG/MinIO/native-door/actual-daemon real-byte lane is implemented and source-reviewed, UNRUN HERE. Named heavy-slot question 6ff17332 remains pending; no stack was started. Existing TALE-617 author owns admitted runtime proof, then TALE-359 must reconcile exact-head proof and ordinary hosted checks. This ACCEPT is not merge readiness, runtime acceptance, native approval, opt-in or deployment authority. Parent #4101/TALE-484 stays open.

No whole-platform suite/workspace tsc, independent TypeScript/SAST rerun, browser/native acceptance or CI evaluation. Pinned local Opengrep was uncached; no installation or SAST pass claimed. No CI rerun/cancel/wait, push, merge, card move or #4275 work. Root's 08:15Z rule applies: a blocking finding overrides ACCEPT until independently closed on the exact head with proof.

Full source reasoning, exact patch/source copies, commands and raw reproduction/static logs: /agent/output/b47c21f0-af19-4cbb-a7c1-480d689dafd4/ (report.md).