docs: post-architecture-deepening audit — trim PLAN.md, fix CLAUDE.md drift

[eschizoid]

Summary

Follow-up to the architecture-deepening squash (#141 / 50c7dbf). Three concerns bundled into one commit since they're all documentation/housekeeping with no code impact.

PLAN.md trim (259 → 172 lines, -87)

• Header rewritten — was pointing at 1.13 / "1.14 in flight"; both shipped, 1.15 is current. Stops re-asserting per-release detail that belongs in git history + CLAUDE.md §19/§20.
• Score table updated for the deepening pass: Discovery 8.5→9, Type safety 9→9.5, aggregate 8.7→8.8 (Result<T> switch is now the only way to get a pipeline outcome, enforced by the compiler).
• "Developer-ergonomics — 1.14 in flight" section collapsed to just the deferred-decisions list — the part that still has forward signal. Shipped items live in commits + §16/§19.
• "Strengths to preserve" section deleted — 100% verbatim duplication of CLAUDE.md core invariants + §3-§14. Two sources of truth for the same content always drift.
• Per-module contents table deleted — restated package contents already recoverable from settings.gradle + CLAUDE.md §9. Kept the ASCII dep diagram and the "Still planned" mini-table.
• Open question Add retry tests #3 ("Confluent SR scope... schedule for 1.14?") removed — answered, shipped, doctrine in §19.

CLAUDE.md audit + fixes

Three parallel Explore agents audited §7/§8/§17, §10/§11/§12, §18/§20 + invariants against current code. Eight substantive drift items fixed:

• §7 vs §8 helper-name inconsistency. §7 said public helpers are on MessageProcessorRegistry; §8 said RegistryFunctions with withConsumerErrorHandling for sinks. Verified public face is on MessageProcessorRegistry; rewrote §8 to match and note the internal delegation as a historical artifact.
• §11 unpark sources. Bullet still named processRecord()'s finally as the third unpark site; it moved to afterRecordComplete() (the dispatcher's onComplete callback) in 1.15.
• §11 markOffsetProcessed-before-errorHandler invariant. Added the one real exception (handleParallelRejection during shutdown).
• §12 byte-level entry points bullet. Said apply / processToSink / processToValue "preserve pre-1.13 contracts." Methods are gone; replaced with an inverted bullet explaining WHY they're gone.
• §12 real-world burn paragraph. Dropped the stale processTypedRecord method name.
• §17 capability table. Four rows had wrong method names:
  • withConsumerProvider → actually withConsumer(Supplier<Consumer<K,byte[]>>)
  • withDeadLetterBundle(...) → actually withDeadLetterQueue(String, KPipeProducer)
  • withOffsetManager(Provider) → actually withOffsetManagerProvider(Function<Consumer<K,byte[]>, OffsetManager<K>>)
  • withRebalanceListener(...) → doesn't exist as a public Builder method; lives on OffsetManager.createRebalanceListener()
• §18 offset-commit bullet. Generalized from batch-only to ALL offset bookkeeping — the deepening pass extended what was a 1.12-batch-only optimization to every processing mode.
• §18 shutdown drain order. Updated to point at the four named phases of the split close() instead of a flat sequence that no longer matches the code structure.
• Coding standards. Added the accumulator-pattern exception to "final var for locals" — one site in the codebase legitimately needs reassignment.

Verified accurate and left untouched: §10 OTel metric table, top-level Core invariants, §13, §14, §15, §16, §19, §20.

Housekeeping

• Removed plans/2026-05-24-fastjson2-migration.md — that migration shipped as 70cb470; the plan file was scratch state.
• Gradle wrapper bump pulled in by the rebase.

Test plan

• ./gradlew spotlessMarkdownCheck — green
• Every modified CLAUDE.md claim re-verified against a specific file + line number before editing
• Net diff against main confirmed: only the two doc files + wrapper + the plan deletion