Restore trim-safe server routing and response dispatch

[quinnj]

Native applications that use HTTP.Router fail JuliaC's --trim=safe verifier on HTTP 2.9. Route registration and the route trie erase handler types, routed handlers return responses whose body type is unknown, and HTTP/2 shutdown calls an erased Function. LeagueEasy's production build reported 48 verifier errors; the new standalone router workload reproduced 15 before this change.

Changes

• Router registration specializes on the handler type, and route segments are stored with concrete types.
• HTTP/2 shutdown state is stored in a concrete struct instead of a closure (all builds).
• The server narrows handler responses to known buffered body types before writing them. Default builds fall back to normal dispatch for any other body type, so they accept every body type as before.
• New opt-in trim_strict_bodies preference. With it enabled:
  • routers call each handler through a per-handler-type C entry point;
  • the narrowed body types (now including servecontent byte ranges over a byte vector) are the only ones accepted; the server answers other body types with a 500;
  • the precompile workload is skipped.
• The server guide documents the preference.

Tests

• A new native router workload (captured closures, middleware, regex and wildcard routes, a byte-range servecontent response, 404/405, shutdown) must compile with zero verifier errors.
• The handler test file also runs in a child process with the preference enabled.
• New tests cover custom request and response bodies in both modes, handler retention across GC, and exceptions thrown from routed handlers.

Validation

• Julia 1.13.1, macOS arm64: the router workload has zero verifier errors and the native executable serves every route, including the 206 range response. Removing the range-body branch makes that request return 500, so the workload covers it.
• The handler tests pass in both modes, and the HTTP/1 and HTTP/2 server tests pass in default mode.
• Full HTTP suite and docs build pass in CI.
• LeagueEasy builds, links, and passes its full backend suite and a native Linux API smoke test with this branch. It has not been deployed.

Co-authored by Codex

AI disclosure: This work was prepared with assistance from OpenAI Codex and Claude Code.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 93.24324% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.83%. Comparing base (52ba567) to head (5db12f1).

Files with missing lines	Patch %	Lines
src/http_handlers.jl	90.69%	4 Missing ⚠️
src/http_core.jl	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1397      +/-   ##
==========================================
+ Coverage   90.80%   90.83%   +0.02%     
==========================================
  Files          31       31              
  Lines       12835    12881      +46     
==========================================
+ Hits        11655    11700      +45     
- Misses       1180     1181       +1     

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