fix(miner-extension): guard the live-fetch sync path against the storage quota

[shin-core]

What

syncRankedCandidatesFromMinerUi (apps/loopover-miner-extension/background.js) — the live-fetch replacement for manual paste — wrote fetched ranked candidates to chrome.storage.local with no size check. The manual-paste flow in options.js already guards against this: it measures the real serialized UTF-8 byte size (via TextEncoder, not JS string .length) against the shared 10 MiB QUOTA_BYTES limit, because an unbounded write can silently fail or leave storage in a partial state (#4863). The live-fetch path never got the equivalent guard, so the same #4863 failure mode could still happen through it.

How

Measure the serialized byte size of the fetched candidates before the chrome.storage.local.set(...) call, against the same 8 MiB ceiling options.js enforces. On an over-quota result, return the function's existing typed { ok: false, error, minerUiUrl } shape (matching every other failure branch and its documented "never throws" contract) rather than writing a partial payload.

Validation

Tests added to the existing test/background.test.ts: an over-quota live-fetch payload is rejected with ok: false and no chrome.storage.local.set call (localSetCalls stays empty), while a within-quota payload still writes normally. Confirmed the over-quota test fails against the unfixed code (it writes the partial payload) and passes with the fix. miner-extension:lint/typecheck and the extension test suite (26 tests) pass; the guard's lines are fully covered.

Closes #7006

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Superagent didn't find any vulnerabilities or security issues in this PR.

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Warning

⏸️ LoopOver review result - manual review recommended

_{Review updated: 2026-07-17 20:22:22 UTC}

2 files · 1 AI reviewer · no blockers · CI green · unstable

⏸️ Suggested Action - Manual Review

Review summary
This PR ports the existing options.js byte-size quota guard to the live-fetch path in background.js, adding an 8 MiB ceiling check via TextEncoder before the chrome.storage.local.set call, and returns the function's existing typed ok:false shape on overflow rather than writing a partial payload. The fix is correctly placed at the actual write site (not a symptom patch), the two new tests genuinely exercise the real code path (one oversized payload that must trip the check, one within-quota payload that must still write), and the described validation (test fails against unfixed code, passes with fix) is credible given the code shown. This closes #7006 as claimed and is a narrowly-scoped, low-risk change.

Nits — 2 non-blocking

• The oversized test builds ~9k objects with a 1KB filler string plus JSON overhead to exceed 8 MiB — this is a reasonable but somewhat fragile way to hit the exact boundary; a comment noting the approximate resulting byte count would help future maintainers avoid accidentally shrinking it below the threshold.
• background.js:14-16 hardcodes 8 MiB separately from options.js's own constant instead of importing/sharing one source of truth, so the two ceilings could silently drift apart in a future edit — consider extracting a shared constant if options.js's guard is in an importable module.

Flagged checks (non-blocking)

• Contributor trust — Contributor flagged for review

Decision drivers

• ✅ Code review — No blockers (1 reviewer)
• ✅ Gate result — Passing (No configured blocker found.)

Context & advisory signals — never blocks the verdict

Signal	Result	Evidence
Linked issue	✅ Linked	#7006
Related work	✅ No active overlap found	No same-issue or scoped active PR overlap found.
Change scope	✅ 20/20	Low review scope from cached public metadata (1 linked issue).
Validation posture	✅ 25/25	PR body includes validation/test evidence.
Contributor workload	✅ 10/10	Author activity: 55 registered-repo PR(s), 33 merged, 0 issue(s).
Contributor context	✅ Confirmed Gittensor contributor	shin-core; Gittensor profile; 55 PR(s), 0 issue(s).
Improvement	✅ Minor	risk: clean · value: minor

Review context

• Author: shin-core
• Role context: outside_contributor
• Public audience mode: oss maintainer
• Lane context: Repository is configured for direct PR review.
• Public profile languages: not available
• Official Gittensor activity: 55 PR(s), 0 issue(s).
• PR-specific overlap: none found.

Contributor next steps

• Keep the PR focused and include validation evidence before maintainer review.

Signal definitions

• Related work = same linked issue, overlapping active PRs, or title/path similarity.
• Change scope = cached public metadata such as size labels, draft state, and review-burden hints.
• Validation posture = whether the PR provides enough public validation/test evidence for maintainer review.
• Contributor workload = public contributor activity and cleanup pressure, not a repo-wide quality failure.
• Contributor context = public GitHub/Gittensor identity context; non-Gittensor status is not a blocker.

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_{🟩 Safe / merged · 🟦 Advisory · 🟨 Held for review · 🟥 Blocked / closed}

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LoopOver approves — the gate is satisfied and CI is green.