fix: guard overlapping ImageGen picks with per-target sequence token (#5944)

[atomantic]

Summary

Fixes the blob-URL orphan when two init-image (or same-slot reference) picks overlap in the EXIF-normalization await: both continuations read the same stale preview ref and each mint a URL.createObjectURL, but only the last setState survives, leaving the loser's url unreachable from state, the clear path, and the unmount sweep.

Per the issue decision, uses a per-target sequence token (pickSeqRef: { init, refs[]}), mirroring the existing statusRequestToken idiom in the same file. Each handler bumps its slot's counter and bails when superseded before calling createObjectURL, so a losing pick never mints at all. The init handler's follow-up dimension probe gets the same staleness guard so a superseded pick can't clobber width/height either.

Closes #5944

Test plan

• Extended client/src/pages/ImageGen.objectUrls.test.jsx (its createImageBitmap deferral harness already existed): two overlapping picks on the init image, and two overlapping picks on reference slot 0 — each asserts exactly one url is created, it belongs to the last pick (photo-2.jpg), it is the live (unrevoked) one, and it is the url rendered.
• Verified both new tests fail without the fix (2 failed / 6 passed) and pass with it.
• npx vitest run src/pages/ImageGen.objectUrls.test.jsx — 8 passed.
• npx vitest run src/pages/ImageGen.probeGating.test.jsx src/pages/ImageGen.federatedTarget.test.jsx src/pages/ImageGen.flux2VenvShare.test.jsx — 9 passed.
• Local reviewer (mtplx, low) was unavailable — no model configured for that backend — so shipping with self-review + tests; reviewer status recorded as review-blocked.

[atomantic]

Review-blocked: the mtplx local reviewer (effort low) returned no verdict — "No model configured for mtplx reviewer — set one on the Settings → Code Reviewers page." Code and tests are otherwise shippable (ImageGen.objectUrls 8/8, sibling ImageGen suites 9/9; new overlap tests verified to fail without the fix), so this PR stays open for human/CI review. Status recorded in the worktree-private review file.