perf(framework): run an analyzer's units concurrently

[elecnix]

Problem

Sessions were the only axis of fan-out. runAnalyzer awaited each unit in turn, so one session could never have more than a single call in flight, however the limits were set.

Measured on a real corpus at 40-way session concurrency:

per-call latency	~1.5 s (flat at both 10- and 40-way)
theoretical throughput at 40 lanes	~26 calls/s
actual	~7.8 calls/s (~30% utilisation)
going 10 → 40 concurrency	only 1.7×

The lanes were not waiting on the provider. Many were doing deterministic work — turn construction, tokenising, matching — and issuing no calls at all, while a session with 100 terms to judge serialised them through one lane for over two minutes.

Raising --llm-concurrency could not fix this: the ceiling was how many sessions happened to be issuing calls at the same instant, not the semaphore. And the degenerate case was worse — --session X alone ran at concurrency 1 regardless of any flag.

Change

Units now run through the existing mapWithConcurrency, bounded by a new unitConcurrency. The analyze command sets it to the LLM concurrency, so a single session can reach the gate by itself.

Layered under the existing limits, not replacing them:

• the global semaphore around the LLM caller is still the only thing deciding provider load
• better-sqlite3 is synchronous, so persistence cannot interleave mid-write
• unitConcurrency defaults to 1, so nothing changes for callers that do not opt in

One honest behaviour change

Fail-fast on configuration faults is preserved, but units already in flight now finish. A bad model spec costs at most one concurrency width of error nodes instead of exactly one — still bounded, against the 113,992 it produced before fail-fast existed. Covered by test.

Test plan

• npm test — 429 passing. New tests/component/unit-parallelism.test.ts pins what could plausibly break under concurrency: one node per unit with no duplicates, idempotent re-runs, the configured limit respected, default-sequential behaviour unchanged, and early stop on a configuration fault.
• node --import tsx test/integration/test-commands.ts — 21 passing.
• npx tsc --noEmit clean.

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