docs(file-ipc): audit legacy file IPC surface and FileStorage consumption

[EurFelux]

What this PR does

Before this PR:

• The legacy file IPC surface (File_* / Fs_* / Open_Path) was undocumented — no single inventory of which channels exist, where they are registered, or who consumes them, and no assessment of which v1 consumption patterns should be re-routed vs abolished under the new IpcApi.

After this PR:

• Adds two audits under v2-refactor-temp/docs/file-manager/ (documentation only — no runtime behavior change):
  • legacy-file-ipc-audit.md — current-state inventory of every legacy file IPC channel (FileManager.ts Group A + ipc.ts Group B), its registration point, backing implementation, and every renderer consumer (file:line). Distinguishes already-migrated IpcApi routes and flags renderer-dead channels.
  • filestorage-consumption-audit.md — for the 25 v1 FileStorage-backed channels, a per-consumer verdict (reroute / abolish / defer) against the target IpcApi (RFC §7.2), with rewrite sketches and a P1–P5 anti-pattern taxonomy.
• Carries a small FileStorage.ts deprecation notice (9 lines) marking the service for v2 migration.
• Records a BLOCKING prerequisite (filestorage-consumption-audit.md §7): the absolute-path type is not unified — FilePath is a type-only template literal, AbsolutePathSchema infers unbranded string, and the two are bridged by as casts (per the handle.ts:59-60 TODOs). This type refactor must land before the audited consumer migration proceeds — hence this PR is a draft.

Fixes # N/A

Why we need it and why it was done in this way

Groundwork for migrating the legacy file IPC surface onto IpcApi. The migration is not a pure transport swap: several v1 consumption patterns (synthesizing throwaway FileMetadata from a raw path, renderer-side temp-file orchestration, type-probe helpers) should be abolished, not re-implemented. Auditing and classifying consumers first prevents carrying v1 anti-patterns into v2.

The following tradeoffs were made:

• Documentation-first + draft: the audit surfaced a blocking type-design issue (absolute-path type not unified). Rather than migrate on top of that debt, the migration is paused and the prerequisite recorded.

The following alternatives were considered:

• A transport-only migration — rejected: it would propagate the existing as-cast path-typing debt from ~3 sites to dozens of consumers.

Links to places where the discussion took place: N/A

Breaking changes

None. Documentation-only, plus an inert deprecation notice on FileStorage.ts.

Special notes for your reviewer

• Draft: blocked on a separate absolute-path type refactor (see filestorage-consumption-audit.md §7), handled separately.
• Quickest scan: legacy-file-ipc-audit.md §1/§4 (inventory) and filestorage-consumption-audit.md §1 (verdict table) + §7 (blocker).

Checklist

• Branch: This PR targets the correct branch — main for active development, v1 for v1 maintenance fixes
• PR: The PR description is expressive enough and will help future contributors
• Code: Write code that humans can understand and Keep it simple
• Refactor: You have left the code cleaner than you found it (Boy Scout Rule)
• Upgrade: Impact of this change on upgrade flows was considered and addressed if required
• Documentation: A user-guide update was considered and is present (link) or not required. Check this only when the PR introduces or changes a user-facing feature or behavior.
• Self-review: I have reviewed my own code (e.g., via /gh-pr-review, gh pr diff, or GitHub UI) before requesting review from others

Release note

NONE