fix(next/font/local): omit font-weight descriptor when no weight is specified

[vjymisal0]

Fixes #2793

Problem

The next/font/local shim's generated @font-face rule always defaulted to font-weight: 400 whenever neither the source nor the top-level options specified a weight:

const weight = sanitizeFontDescriptorValue(src.weight ?? options.weight ?? "400") ?? "400";

For variable fonts with a wght axis, this clamps the font face to the regular (400) instance. Elements styled with e.g. font-weight: 600 still compute to 600 in the CSSOM, but the browser renders the 400 instance of the variable font instead of interpolating to 600, because the @font-face rule pins the descriptor.

Fix

packages/vinext/src/shims/font-local.ts: only emit the font-weight descriptor when a weight was actually specified on the source or top-level options. font-style keeps its existing "normal" default, matching the expected output in the issue.

This matches Next.js, which only emits font-weight in the generated CSS when weight/defaultWeight is provided (see packages/font/src/local/loader.ts and the Turbopack next_font/local/stylesheet.rs implementation, both linked from the issue).

Test plan

• Added two tests to tests/font-local-transform.test.ts:
  • omits the font-weight descriptor when no weight is specified (issue #2793) — asserts no font-weight in the generated CSS and that result.style.fontWeight is undefined when no weight is given.
  • still emits font-weight when a weight is specified on a single non-object source options — regression guard for the existing explicit-weight path.
• Verified RED: reverted only the source fix (kept the new test) and confirmed the new test fails against the old code with font-weight: 400 present.
• Verified GREEN: pnpm test tests/font-local-transform.test.ts — all 43 tests pass.
• pnpm test tests/shims.test.ts -t "font" — all font-related tests in the broader shims suite pass (19 passed, rest unrelated/skipped).
• vp check (format + lint + typecheck) passes on the changed files.

Scope note: this only touches font-local.ts — I checked font-google-base.ts for the same pattern and it doesn't hardcode a font-weight default, so it isn't affected.

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