Fix 0.5.4 backend regressions and builtins

[loreste]

What changed

• Fixes 0.5.4 C backend regression: MakoString passed as MakoIntArray in large files #34 by preserving raw and C-mangled local type metadata so reserved names like bind/read/index still lower len(string) to mako_str_len in the C backend.
• Fixes OpaqueHandle: complete the type so opaque values can go in collections #35 by completing OpaqueHandle type checking, parsing, C codegen, and native lowering for erased opaque handles in collections.
• Fixes Missing builtins: base64url, jwt custom header, cmap composite keys, udp reuseport addr #36 by exposing/wiring base64url, custom ES256 JWT headers, CMap composite/int helpers, nb_udp_bind_reuseport_addr, and tls_server_pool aliases across C/native/runtime/docs.
• Adds regression tests and updates builtin/status/performance/product docs.

Validation

• cargo check
• cargo build
• cargo build --release
• cargo test: 146 passed
• mako test examples/testing/c_reserved_string_len_issue34_test.mko --backend c/native
• mako test examples/testing/opaque_handle_issue35_test.mko --backend c/native
• mako test examples/testing/issue36_builtins_test.mko --backend c/native
• mako test examples/testing/opaque_collection_test.mko --backend c
• mako test examples/testing/udp_reuseport_c_backend_test.mko --backend c
• Madis repro: tests/maf_headers_test.mko --backend c
• git diff --check

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[loreste]

Changes landed on main via direct commits (6a87748, 530ff23, 5bfb008). All three issues (#34, #35, #36) are closed. TLS reload cert mutex fix applied for Linux CI. Tag v0.5.4 updated.