feat: add compose image transforms and explicit file export

[danielkov]

Summary

Add hidden image_rotate, image_crop, image_resize, and export_file callables that consume session-authorized managed File references. A complete pipeline runs in one compose invocation; only final returned references deliver pixels.

Depends on #126. This PR is intentionally based on feat/compose-files-phase-1, not main, and implements phase 2 of the compose-managed-files plan.

Impact

Transforms create immutable RGBA8 PNG snapshots after EXIF normalization. Explicit export preserves exact stored bytes and refuses existing destinations; interrupted exports may retain a partial file. PNG compressed profiles and compressed/international text metadata are rejected before decoding to bound ancillary expansion.

Technical details

• Define clockwise rotations, nine crop anchors, integer rounding, and contain/cover/stretch resizing with Triangle filtering.
• Reuse existing session authorization, durable storage, and compose delivery, including provider user-image fallback and replay.
• Enforce decode, pixel, resize-scratch, estimated live-work, and encoded-output limits without changing dependencies or the persistent File envelope.
• Export with process OS authority using disk-only create-new, private Unix permissions, and a documented cancellation/commit boundary. Export receipts contain path, byte count, and status rather than a File marker.

Subagent multimodality and generalized image presentation remain separate phases.

[danielkov]

Local checks passed for 24746c29941e4489f87f14dcd7ec5e3d70c64532:

• cargo fmt --check
• scripts/tests/test_next_release_version.sh
• cargo clippy --locked --all-targets --all-features -- -D warnings
• RUST_TEST_THREADS=2 cargo test --locked: 1,148 library tests, 33 binary tests, and all integration suites passed. Five library helpers are ignored by the ordinary harness; the managed-file subprocess helpers are explicitly invoked by their parent tests.

Coverage includes all eight EXIF orientations, nine crop anchors, aspect/fit rounding, scratch/output-budget rejection, session authorization and immutable bytes, exact no-clobber exports, a real OS file-size-limit failure retaining partial output, unused effect execution, final-return-only image delivery, and replay after source/export deletion.

CI and actual current-head GitHub reviewer approval are tracked separately. This stacked PR depends on #126; no merge or auto-merge is authorized.

[kit-code-agent]

One correctness issue: transparent PNG resizing filters straight-alpha pixels directly, producing dark halos or color bleeding at transparent edges.

[kit-code-agent]

No issues found. The changes look good to merge.

[kit-code-agent]

No issues found. The changes look good to merge.

[kit-code-agent]

No issues found. The changes look good to merge.

[kit-code-agent]

No issues found. The changes look good to merge.