refactor(http2,http3): pull repeated wire and header code into inline helpers

[gg582]

Refactor only (one small fix noted below): repeated wire/header code in the HTTP/2 and HTTP/3 paths moves into static inline helpers.

HTTP/2

• src/net/http/h2_wire.h (new, private): h2_put_u24/u32/u31, h2_get_u16/u24/u32/u31. Replaces ~20 hand-written shift/mask sequences:
  • frame header write (h2_make_frame_header) and both frame-header parsers;
  • GOAWAY, RST_STREAM, WINDOW_UPDATE and PUSH_PROMISE payloads;
  • both WINDOW_UPDATE parsers and SETTINGS parsing;
  • the flow-control module's WINDOW_UPDATE builder (http2_flow_control.c).
    The 31-bit variants hold the reserved-bit rule (sent as 0, ignored on receipt) in one place.
• h2_env_long(): the five env knobs (idle timeout, GOAWAY grace, RST burst/rate, batch bytes) share one parser. getenv("CWIST_...") stays at each call site so the runtime-knob gate (docs(config): document every CWIST_* runtime variable and gate it (#340) #360) still sees them; defaults and minimums are unchanged.
• h2_now_ns() backs h2_now_ms() / h2_now_us().

HTTP/3

• src/net/http/h3_hdr_out.h (new, private): h3_hdr_out_add() / h3_hdr_out_add_str() append one field to an lsxpack header set (name, 2 spare bytes, value) with slot and buffer bounds checked once. Used by server response headers, server push, and client request headers — 13 copies of the same 6 lines.
• h3_json_error(): status + JSON body + Content-Type for five protocol error responses (malformed request ×2, payload too large, incomplete sequenced body, too early).
  • Small fix: the old code added Content-Type only if (st->res->headers). A fresh response starts with headers == NULL, so these errors went out without a content type. They now always carry application/json.

Diff: 5 files, +101 / −294, plus the two new headers.

Checks

• WERROR=1 build.
• test_http2, test_http2_flow_control, test_http2_prebuffer, test_http3, test_webtransport, test_webtransport_client_api, test_grpc, test_grpc_append_error, test_grpc_channel, test_grpc_client, test_grpc_stream: all pass.
• scripts/ci/h2spec_gate.sh: no new failures vs the baseline (146 tests, 103 passed — same as dev).
• clang-format applied to the changed regions only; formatter churn in unrelated code was reverted.