perf (speculative): six micro-candidates — evaluate each before any work

[JavaGT]

Filed by the Account-2 read-only amplifier (2026-09-16, HEAD f635e62). Six low-stakes candidates from the hot-path/simplification sweep. Each is evaluate whether it is worth doing — implementation is explicitly NOT the todo. All evidence coordinator-verified (read + rg).

1. CookieJar's keep-alive agent is never destroyed — created per fetchOpenbook (src/openbook.mjs:299; agent at :341); rg finds no .destroy. One idle socket accumulates per loan in archive --all; impact bounded by the explicit process.exit at end of run. Evaluate: destroy in a finally, or accept and close.
2. Legacy eager {body} spine shape is production-dead — archive-read.mjs:121-126 emits only {data} thunks; only tests construct {body} (test/ram-thunk.test.mjs:83-93); epub.mjs:251-252 keeps the branch alive for them. Evaluate: migrate the tests and retire the branch.
3. collect()'s textDone sentinel looks redundant — buffer.toString('utf8') never returns undefined; only the JSON variant needs the guard (src/http.mjs:56-77). The contract is test-pinned (test/http.test.mjs:18), so change only with tests in hand. Evaluate.
4. Cover download is chained behind thunderP although loan.coverUrl is synchronously available (src/archive.mjs:57, src/archive-read.mjs:74) — the perf: start thunder/cover at archive entry — catalog RTTs are serialized after the open chain though they depend on nothing from it #18 entry-time window must then cover thunder+cover serially. Decoupling risks re-fetching a "better" cover late (behavior change). Evaluate against perf: start thunder/cover at archive entry — catalog RTTs are serialized after the open chain though they depend on nothing from it #18's intent.
5. TOC→part mapping does Array.indexOf per TOC entry — O(toc×parts) where a Map is the standard primitive (src/archive.mjs:122-125); trivial at current sizes (buildNav already uses a Set). Evaluate/decline.
6. Entry extension split+lowercase runs twice per asset — encodeEntry (src/epub.mjs:42) and mimeOf (:226). Nano alloc noise. Evaluate/decline.