ci: delete the -m32 ILP32 approximation gate; the real wasm build is required (#1274)

[InauguralPhysicist]

Closes #1274. First step of #1275 (gate consolidation).

What: deletes tools/ilp32_syntax_check.sh (3,123 lines) and suite [99i3]. It compiled the playground's translation units with host clang -m32, an approximation of emcc's wasm32 target that #1255 showed can pass while the real build fails (i386 aligns double to 4, wasm32 to 8). Since #1267 the real web/build.sh under emcc runs on every PR, and since #1273 playground (real emcc wasm32 build) is a required check. The approximation guards nothing the required check doesn't.

Removed with it:

• the [99i3] runner section
• its werror SCRIPT_AUDITS entry and floor row, plus the ~30-line floor history comment
• the doc prose describing its internals (docs/PERFORMANCE.md, docs/CI.md)

Floors lowered deliberately (each gate asks for this in the same commit when coverage is removed):

• child_exit_check CHILD_SITES_FLOOR 128 → 126 (the section's two child invocations)
• section_plan SKIP_ROUTED_FLOOR 26 → 25 (the section's section_skip site)

Doc fix alongside: CI.md said the wasm check was not yet in the ruleset. That was stale since #1273; it now says the check is required.

Net: +33 / −3,501 lines.

Local verification (4GB dev box):

• make precheck: green after the floor change
• werror_switch_check.sh + --selftest: green
• section_plan.sh --selftest: 50 checks, 0 failures
• docs_claims_check.sh: green
• full suite: the only red not caused by this change was issue_labels_check on nightly lane: macos-15-intel / valgrind-full failures #1168, whose labels were missing. Unrelated GitHub state, the precheck: parallelise core_ext_boundary_check (the precheck's critical path, 17–35 s) #1279 class; nightly lane: macos-15-intel / valgrind-full failures #1168 is now labeled, and moving that check out of the suite is part of tooling: gate scripts grew 59x in 7 weeks (592 → 34,804 lines; half the runtime) — freeze, consolidate, budget #1275.

🤖 Generated with Claude Code

[codspeed]

Merging this PR will not alter performance

✅ 12 untouched benchmarks

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_{Comparing ci/retire-ilp32-gate-1274 (d2d5ffd) with main (3130a17)}