Fix 4 emit-bug classes: async-await, let-swallow, %-modulo, camelCase-export mangle

[tompassarelli]

Four classes of incorrect JS emit. Each passes beagle syntax but produces output that only fails at runtime. This PR fixes or guards each.

1. async-IIFE-await — emit correctness (678bbd1)

A try/loop/doseq containing (js/await …) compiles to an async IIFE (async () => {…})(). When bound in a let without an enclosing js/await, it was emitted without await, so the binding held a pending Promise that downstream code read synchronously.
Fix: emit-js awaits async IIFEs in value/statement position (await-async-iife, applied at binding-value and expr-stmt sites). Tail position is left un-awaited; the helper matches only the exact (async () => prefix, so it never double-awaits an explicit js/await or an inline f(await g()).

2. let-swallow → bare name; — guard (004d291)

A let binding whose value form is one paren short absorbs the following name value pair as body forms; the swallowed binding name emits as a bare name; statement, causing a ReferenceError at load. Net parens balance, so beagle syntax reports ok.
Fix: check.rkt raises swallowed-binding (E020) when a non-final body statement is a bare symbol that resolves to nothing, with a message pointing at the enclosing let. Tail-position bare symbols (legitimate returns) are not flagged.

3. % as call head → undefined _pct — guard (2f7a441)

(% a b) emits _pct(a, b) because % is the #() anonymous-arg shorthand. Outside a lambda this is an undefined call.
Fix: parse.rkt rejects % as a call head with an error naming rem/mod. % inside #(…) is unaffected.

4. camelCase export name mangle — lint (7b5b6d5)

kebab names mangle to snake_case; camelCase emits as-is. An exported camelCase name referenced cross-module in kebab mangles to a different identifier and is undefined in the bundle.
Fix: lint.rkt lint-export-naming warns on camelCase JS exports and names the kebab form. It runs in lint-program!, so it fires at build and plain check, and is counted by count-lint-warnings so the --agent path reports it.

Verification

• active suite 1377/1377
• gated tier: no new regressions vs baseline (emit-js 127/131, js-quote 67/71, jst 24/33, js-exec-oracle 4/5 — pre-existing)
• downstream consumer compiles clean (79/79 files), integration 42/42