Safe cleanup backlog from review (write-only fields, dedup helpers, test-only dupes) #570

Summary

Remaining "safe" dead-code / dedup / doc cleanups from the multi-agent review. These are low-risk and behavior-preserving but were left out of the stabilization PR because each is a multi-file or test-heavy edit (write-only field removals that thread through dep contracts + ~20 test fixtures, helper extractions into another module, etc.) — not the surgical, single-file changes that batch cleanly. Internal only; no consumer-facing change.

Already done in the stabilization PR (for context): removed dead exports SegmentCacheProvider/CacheConfig/CachedEntryResult/CacheStatus/RouterDependencies/LoaderRevalidationResult/OutletProvider, added missing re-exports (PrerenderPassthroughResult, OriginCheck*), removed the navigation-store singleton test-island, de-duped paramsEqual + observeStreamedHandler.

Backlog

• RouteMatchResult.optionalParams / TrieMatchResult.optionalParams written everywhere, read nowhere. All-or-nothing removal: also drop TrieLeaf.op + its build in src/build/route-trie.ts:162-178, and migrate pattern-matching.test.ts assertions that read result.optionalParams. One coordinated commit.
• MiddlewareEntry.mountPrefix is a required write-only field. Assigned at router.ts:344 + hardcoded null at middleware.ts:700/rsc/helpers.ts:210; read nowhere (scoping folds into fullPattern before the entry is built). Make optional/remove; ~20 mountPrefix: null test-fixture lines to update.
• loaderPromises param threaded through revalidation but never read. Forwarded once at revalidation.ts:1390 into a function that ignores it. Removal threads through RouterContext dep type, segment-wrappers, resolveAllSegmentsWithRevalidation, the segment-resolution.ts call site, and segment-resolution-parallel-loading.test.ts.
• middleware-cookies.ts parseCookies/serializeCookie are test-only duplicates of a private byte-for-byte copy in server/request-context.ts. Have request-context import these (removing its dupes), or delete + drop the middleware.ts:39 re-export + orphaned test blocks.
• src/handles/index.ts barrel has zero importers. Delete (verify MetaTags is reachable elsewhere first — it may only be exported via this barrel) + update feature-file-map.md:33.
• MiddlewareCollectableEntry re-export in middleware.ts — repoint middleware.test.ts:14 to import from ./middleware-types.js, then drop the re-export for a fully-minimal block.
• Duplicated context-vars-non-empty predicate (keys + symbols) across two prerender entry points — extract hasContextVars(o) next to contextSet in src/context-var.ts and use it in both prerender-collection.ts and the second site.
• ctx.reverse() fallback in createMiddlewareContext — misleading message ("route map was not provided to middleware context") and (name: string)-only signature, narrower than the public (name, params, search) ScopedReverseFunction.
• Static-handler Response not routed through handleHandlerResult in renderStaticSegment — a Static handler returning a Response serializes a corrupt artifact instead of failing loudly. Add the if (raw instanceof Response) throw guard (behavior change on a type-forbidden misuse path; add a test).
• Docs: tree-structure.md inTransitionScope description omits that the key predicate also covers error/notFound segment types (defensive); promote the document-cache tag-union timing-gap caveat (document-cache.ts:45-53) to a committed design-doc "Open Problems" entry.

Acceptance

Each item: grep-verified safe, gate green (typecheck / 3650+ unit / lint / format), no public-API change. Batch into one or two commits.