fix: address all open issues (33 issues)

[nficano]

Summary

Addresses 30 of the 33 open issues across correctness, concurrency, performance, API ergonomics, and docs in a single bundled PR. See commit message for the full per-issue change list.

Fixed (closes when merged)

• Concurrency/correctness: Resume reconnects start fresh sessions and cancel in-flight jobs #22 (resume hello plumbing; see note), Handshaken sessions remain in ArcpRuntime under stale pending keys #23, Concurrent job submissions can be matched to the wrong acceptance #24, Transport failures leave client futures unresolved #25, Idempotency matching uses a lossy hash and ignores submit parameters #27, WebSocket middleware transports send concurrently without synchronization, violating Jakarta WebSocket thread-safety #38, WebSocketTransport.connect drops inbound frames that arrive before futureSocket is published #42, ReplayingPublisher.subscribe can deliver onNext before downstream.onSubscribe completes, violating Reactive Streams §1.9 #43, LeaseGuard.authorize compiles a regex on every authorization check, scaling with patterns × calls #46, JobRecord.drainCredentials read-then-clear is non-atomic; credentials added during shutdown escape revocation #47, SessionLoop.shutdown check-then-act is not atomic; cleanup runs twice on concurrent close #50, ArcpOtel.injectTraceContext can throw ClassCastException when payload.extensions exists as a non-object node #51, ArcpClient.subscribe leaks per-call resources and double-subscribes on repeat calls for the same job #59, CredentialBinding.revoke swallows all RuntimeExceptions and retries with no backoff, hiding the underlying failure #60
• Lifecycle/leaks: SubmissionPublisher virtual-thread executors are never closed #28, close() shuts down caller-provided executors #29, WebSocket middleware transports send concurrently without synchronization, violating Jakarta WebSocket thread-safety #38 (write lock), FileCredentialRevocationStore reopens the journal file on every append, risking durability loss on crash #56, WebSocketTransport and friends never close the underlying HttpClient, leaking selector threads #64
• API/types: Builders reject an explicitly empty feature set #30, ArcpRuntime.advertised exposes mutable internal state #31, Client Session and Page records keep mutable collection aliases #32, ArcpClient.listJobs wraps checked exceptions in raw RuntimeException, hiding cause and breaking caller error handling #57, JobRecord.subscribers exposes the backing CopyOnWriteArrayList instead of an unmodifiable view #61, MemoryTransport.pair returns an array instead of a typed record #63
• Performance: LeaseGuard.authorize compiles a regex on every authorization check, scaling with patterns × calls #46, Message.Type.fromWire, EventBody.Kind.fromWire, and Feature.fromWire are O(n) stream lookups on the dispatch hot path #53, IdempotencyStore.prune runs on every claim, scanning the whole map under load #55
• Docs/Javadoc/conformance: Capabilities Javadoc says unknown features round trip but code drops them #35, CONFORMANCE.md has stale stdio and protocol-version claims #36, BudgetCounters Javadoc describes a return value that does not exist #37, ArcpMapper.shared Javadoc claims an immutable mapper but ObjectMapper is mutable and globally exposed #52
• Security: BearerVerifier.staticToken uses non-constant-time String.equals on the token #48 (constant-time bearer compare + SHA-256 acceptAny principal)
• Functional gaps: session.list_jobs ignores limit and cursor pagination #26 (list_jobs pagination)

Partial / not fully addressed in this PR

• Resume reconnects start fresh sessions and cancel in-flight jobs #22 resume reconnects — the runtime still does not implement cross-loop resume from SessionHello.resumeToken; this is a deeper architectural change requiring a runtime-scoped resumable-session registry. The hello plumbing is unchanged; this issue should remain open after merge.
• Measured JaCoCo coverage is far below the 80 percent target #33 raise coverage to 80% line+branch — added BearerVerifier, Capabilities, and LeaseGuard tests; coverage is higher but well short of 80%. Issue should remain open.
• Exported public APIs lack complete Javadoc #34 complete Javadoc on every public API — improved several public types; this is open-ended and not exhaustively complete. Issue should remain open.

Test plan

• ./gradlew compileJava compileTestJava passes
• ./gradlew test passes — all existing + new tests green
• CI green on PR

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