fix(compiler): allow bare side-effect imports of ambient-only modules

[techfreaque]

Problem

A bare side-effect import — import "somepackage/dist/style.css";, with
no default, named, or namespace binding — is a no-op in every JS runtime
that isn't asked to bundle CSS, including scriptc's own compiled
binaries (there is no browser to apply a stylesheet side effect to).
When the imported specifier resolves only to an ambient declare module "*.css" (or similarly-shaped) type surface — i.e. there is a type
declaration but no compilable runtime module behind it — the preflight
import-statement pass in packages/compiler/src/frontend/program.ts
still hard-errors on it:

error SC1010: the '<spec>' module (it exists only as an ambient 'declare
module' type surface — there is no runtime module to import) is not
supported yet

This is a near-universal pattern for any component library that ships
its own stylesheet, and it blocked the containing file from being
analyzed at all.

Root cause

The bare-import case was treated identically to a bound import (import x from "spec", import { x } from "spec") of the same kind of module.
For a bound import that's correct — the program expects a real binding
that doesn't exist. For a bare side-effect import, nothing is bound, so
there is nothing later code could observe being missing.

Fix

In preflight7 (packages/compiler/src/frontend/program.ts), before
the existing ambient-module fence fires for a bare import specifier that
resolves to types only, add a guard: if the import statement has no
import clause at all (stmt.importClause === undefined, i.e. a pure
import "spec";) and the specifier is only ambient-declared
(ambientDeclared(spec), an existing helper already used by this
function's diagnostic messages), skip the statement instead of fencing
it. Bound imports of the same module still fence exactly as before.

Verification

• cd packages/compiler && node node_modules/typescript5/bin/tsc -p tsconfig.json — 0 errors.
• Existing frontend/preflight test suites (packages/compiler/test/ts7,
  tests/harness/diagnostics.test.ts) pass identically before and after
  the change (compared against the unmodified base) — no regressions.
  The tests/diagnostics/pkg-import-fences fixture, which asserts bound
  ambient-module imports still fence, is unaffected since all its
  imports bind names.
• Manual repro via the compiler's analyze() API: a declare module "*.css"; ambient declaration plus a bare import "foo/dist/style.css";
  now analyzes with zero diagnostics, while import styles from "foo/dist/style.css"; against the same declaration still produces the
  SC1010 diagnostic.

[vercel]

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