feat: t27_check -- parse and typecheck .t27 source, with a fix hint for foreign syntax

[gHashTag]

Closes gHashTag/t27#10073

Part of the epic gHashTag/t27#10066 (learnability: parser hints for foreign syntax). Stacked on #4 (t27f-deck), which carries wasm/BUILD; the rule table is gHashTag/t27#10080.

What

A new tool, t27_check: give it .t27 source, get back whether it parses and typechecks and, for each
diagnostic, the line, the source line and a short "write this instead" hint for foreign syntax. It runs offline.

t27_check({source: "module M;\npub fn f(a: u8) -> u8 {\n    return (a as u3);\n}\n"})
{ ok: false, errors: 1, warnings: 0, diagnostics: [{ severity: "error", line: 3,
  message: "parse error in fn 'f' near line 3: unknown cast target type `u3`; ...",
  hint: "Casts reach only bool u8 ... usize f32 f64: there is no `as u3`. Keep a wider type and mask the value, e.g. `x & 7`.",
  source: "    return (a as u3);" }] }

• wasm/explorer.wasm: the bootstrap parser and typechecker (gHashTag/t27 bindings/wasm-explorer), copied
  unchanged, 774 KB. It decides ok or not; the server adds no verdict.
• wasm/check_hints.wasm + wasm/check_hints.mjs: generated from specs/t27mcp/check_hints.t27 by
  t27c gen-c + zig cc and t27c gen-js (the string parameters of the spec arrive as const char*, so no
  hand export glue is needed). Built on the Railway lab; commands and sha256 in wasm/BUILD.
• check.mjs (26 lines) is glue: it moves bytes between the two wasm modules. A message the spec does not know
  gets no hint, and the compiler's words stand alone.

Hand-written JavaScript added: check.mjs 26, server.mjs 11, smoke.mjs 11 = 48 lines (cap: 40 per file, 80
per PR).

Evidence

• npm run smoke: the four new t27_check checks pass (clean source, as u3 with its line and source, in as a
  name, and a negative control: an arity error gets no hint). The run's one failure,
  backendFailures tally matches the specs blaming a backend (737 counted vs 113 blaming), is in the live atlas
  data and in code this PR does not touch; it is the kind of tally logic the epic's last item moves into .t27.
• 200 checks of a failing source take 58 ms in total.
• Spec side: 13 of 13 tests, 0 vacuous, negative control fails as it should (spec(t27mcp): check_hints -- compiler message to a short fix hint for foreign syntax t27#10080).

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