fix: trim-compatibility for Router registration, the precompile workload, and one error path

[quinnj]

Summary

• specialize router registration on concrete handler and middleware types
• preserve concrete route segment storage
• allow static builds to disable HTTP's runtime precompile workload
• keep HTTP/2 proxy errors trim-resolvable
• narrow client request copies and server response bodies at trim-sensitive boundaries
• add focused client and server trim-compilation coverage
• flush live SSE events before producer close

Why

Julia does not specialize function-valued arguments that are only passed through. Router registration therefore converged on widened handler types, leaving middleware wrapping and parametric route construction as runtime operations that juliac --trim=safe could not resolve.

The same real application build exposed a few later widening boundaries in request copying and response serialization. This PR uses concrete type parameters at registration and explicit narrowing at those boundaries. It does not change HTTP dispatch behavior or the public request and response model.

The package precompile workload starts real servers and requests. Static compilation must be able to disable that workload, so HTTP_PRECOMPILE_WORKLOAD=0 provides an explicit build-time opt-out.

Julia 1.13 RC1 does not yet preserve jl_new_task starter reachability during safe trimming. Task-backed fixtures therefore remain strict on Julia 1.12 and on patched Julia 1.13+ runtimes that set HTTP_TRIM_TASK_PATCHED=1; vanilla 1.13+ still runs every non-task trim fixture. This keeps the package matrix honest without treating a compiler limitation as an HTTP verifier regression.

SSE response reads previously asked the producer-backed buffer to fill the entire destination, so a live event could remain buffered until the producer closed. The server path now forwards currently available bytes immediately while preserving any remainder for the next read.

Validation

• the complete local HTTP.jl test suite passes, including all 78 trim tests
• focused HTTP/1 and HTTP/2 SSE tests prove the first event arrives before the producer closes
• HTTP client, handler, public API, HTTP/2, precompile-workload, and documentation tests pass locally
• task-backed and strict-body trim fixtures pass locally on Julia 1.12.6
• the compiler-limit gate behaves as expected on Julia 1.13 RC1
• the exact PR head passes the full upstream Linux, macOS, Windows, Julia stable, and Julia prerelease matrix
• Codecov project and patch checks pass
• LeagueEasy's 30-plus-package server graph compiles with a patched Julia nightly and juliac --trim=safe with zero verifier errors attributable to HTTP
• the resulting trimmed LeagueEasy binary is deployed and serving production traffic

Co-authored by Codex

[codecov]

Codecov Report

❌ Patch coverage is 89.14286% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.27%. Comparing base (25388ae) to head (550854c).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/http2_server.jl	73.91%	12 Missing ⚠️
src/http_handlers.jl	89.79%	5 Missing ⚠️
src/http_core.jl	95.55%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1330      +/-   ##
==========================================
+ Coverage   88.04%   88.27%   +0.22%     
==========================================
  Files          30       30              
  Lines       11849    12028     +179     
==========================================
+ Hits        10433    10618     +185     
+ Misses       1416     1410       -6     

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[quinnj]

Both review points addressed in 26c9b73:

Consolidated narrowing helpers — the per-site isa chains are gone. Two @inline helpers in http_core are now the single source of truth for the covered type set:

• _with_body_narrowed(f, body) — the body shapes (nothing/String/SubString{String}/Vector{UInt8}/EmptyBody/BytesBody{Vector{UInt8}}/AbstractBody residual/fallback). Each branch re-narrows so f's call is statically dispatched; sites pass a closure (or a multi-method function like the new _write_h2_body_shape! shapes) with their site-specific logic. Used by _response_has_body, the h1 chunked/exact writes, h2 emptiness + byte normalization + streaming writer, and _body_close_any!.
• _with_response_narrowed(f, response) — the Response{B} equivalent, shared by both boundary dispatch shims.

Adding a supported shape is now one branch in one place.

Trim-compile tests — turns out master already had the trim harness + 13 client workloads, so I added two server-write workloads to it (both zero-tolerance: compile with 0 verifier errors AND run the binary with wire-level assertions, under stock juliac):

• http_trim_server_response_shapes.jl: serves every supported body shape and verifies each over a raw exchange — this exercises the widened-response shims, both narrowing helpers, and the h1 write chains end-to-end.
• http_trim_server_router_registration.jl: Router construction + every register! form (middleware wrap, wildcard/param paths, do-block form). Serving through the router is deliberately not covered yet — that's the handler-table dispatch frontier your router-dispatch design will address; the workload comment marks where the serving case should go once it lands.

Validation: both new workloads pass locally (HTTP_TRIM_ONLY=… runs them in ~10s each thanks to juliac caching), h1+h2 server testsets green, and the downstream full-server graph's verifier count is unchanged by the refactor.

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