Refactor std.* call signature table in checker

[kylejryan]

Summary

Collapses the three parallel strcmp(...) chains in native/zero-c/src/checker.c — std_call_return_type, std_call_arg_count, std_call_arg_type — plus the small std_http_error_code helper into a single sorted lookup table over a new StdSig record, looked up via bsearch. The per-helper signature now lives in exactly one place (native/zero-c/src/std_calls.inc).

Why

The old shape stated each std.* helper's signature three times across three functions, in three (manually maintained) orderings. Updating one and forgetting another was a silent typechecker bug. Each typecheck of a std.* call also paid for up to three full linear strcmp chains (~130 entries each) plus two heap-allocated ZBufs for the dotted name.

What changes

• native/zero-c/src/std_calls.inc — new file. One row per helper, declared with STD_CALL_<N>(name, return_type, args...). The arity is structurally fixed by the macro name, so arg_count cannot drift from the populated argument list. NULL is used for argument slots whose type is not enforced at the table level (matching prior behavior for std.fs.readAll, std.json.parse{,Bytes}, std.mem.{allocBytes,byteBuf,get,eqlBytes}).
• native/zero-c/src/checker.c:
  • Replaces the four functions with a single StdSig table, a std_call_lookup binary search, and three thin accessor functions preserving the existing call-site signatures (modulo std_call_return_type switching from const Expr * to const char * for consistency with the other two accessors — the two call sites are updated).
  • Adds a debug-only __attribute__((constructor)) (check_std_sigs_sorted) that asserts the table is sorted at startup. Release builds (NDEBUG) skip it entirely.
• Includes std.http.error* constants (errorNone, errorConnect, …) explicitly in the table; they were previously implicit via std_http_error_code fall-throughs in two of the functions. ir.c keeps its own independent copy of ir_std_http_error_code and is untouched.

Net diff: +228 / -437 lines.

Performance / space

	Before	After
Lookups per typechecked call	up to 3 linear strcmp chains	up to 3 bsearches
Comparisons per lookup	~130 strcmps worst case	~7 strcmps (⌈log₂ 148⌉)
ZBuf allocations at the hot call site	2 per call	1 per call
.text for the lookup tables	~15 KB across three functions	~75 lines of code + ~2 KB of .rodata

The Makefile already picks up src/*.inc via its INCLUDES glob; no build changes required.

Test plan

• make -C native/zero-c — release build clean with -Wall -Wextra -Wpedantic -Os -std=c11
• Byte-for-byte snapshot of zero check against all 182 .0 files under examples/ — zero diff vs. pre-refactor
• pnpm run native:smoke — pass
• pnpm run reliability:smoke — pass
• bash scripts/sanitizer-smoke.sh — ASan + UBSan clean
• pnpm run test:zero — 10/10 CLI tests pass

Notes / follow-ups (not in this PR)

• The three accessor functions could be inlined at their five call sites in checker.c to do a single std_call_lookup per typecheck instead of up to three, but that touches the large EXPR_CALL block and was kept out of scope here.
• Once this lands, ir.c's ir_std_http_error_code could be deduplicated against the same table, but it's a separate module and out of scope.

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