Complete attachment contribution integration with bounded staging and safe metadata

[tt-a1i]

Context

Review of #365, #380, and #388 while integrating existing community contributions found overlapping, currently unwired attachment modules. Preserve their useful admission, command binding, and cleanup work, but integrate them only with a working upload/composer/Pi prompt path.

Confirmed blockers at reviewed heads

• fix(web): validate attachment filename types #380 (5bf88a1bd71e2dfd5875e74394e6d9c30b43891d): inherited MIME names such as toString pass the regular-object lookup, then .some throws. Use an own-key lookup or Map and test hostile metadata.
• feat(web): add bounded attachment admission rules #365 (ad0e9fd107bbedeb8e84b9189f30c6deca1d6a2d) and fix(web): validate attachment filename types #380 independently add the same module; their ASCII filename policy rejects ordinary Chinese display filenames.
• feat(web): add attachment staging lifecycle #388 (be9eab7c2732df8d2a40254a35e8f382bcded973): a local isolated probe accepted 10 zero-byte stages with maxStagedBytes=1 and maxAttachments=1. Active stage count needs its own bound.
• The same feat(web): add attachment staging lifecycle #388 probe showed dispose removes the staging directory but leaves its adjacent .owner marker.

Completion criteria

Resolve the alternate validators, preserve Unicode display names without making them storage paths, bound both stage count and bytes, and prove owner-marker cleanup. Wire a real user flow and cover consume/discard/dispose before treating this as shipped attachment support. Credit @seekskyworld and @testikun for the adopted contributions.

Related integration tracking: #440. These are review findings, not claims about installed runtime behavior or fixes already shipped.