fix(symbols): track unrenderable variants per-variant so the archive can build

The weekly snapshot has failed since 2026-06-14. The Swift half of that was
fixed earlier this week; today's scheduled run got all the way through the
toolchain, the dylib build and the full corpus sync, then died in
`Build archives`:

  SnapshotIncompleteError: 4 pre-rendered SF Symbol variants missing
  (public/square.and.arrow.up ultralight/{small,medium,large}, thin/small)

macos-26 draws that symbol at every other weight, so v27's whole-symbol
`render_unsupported` flag never applied: mark-unrenderable.js only set it when
EVERY variant failed, deliberately leaving partial failures to 'stay loud'. But
nothing on the host can ever produce those 4 renders, so 'loud' meant the
completeness gate counted 4 permanently-missing files and refused to build any
snapshot, forever.

Make the concept per-variant (v28 `sf_symbols.unsupported_variants`, a JSON
`"<weight>/<scale>"` array):

  - the prerender loop records the exact failing set for a partially-drawable
    symbol, and still sets the v27 flag when no variant draws at all;
  - the gate skips precisely those variants and keeps counting everything else,
    so a real regression still fails the build;
  - both are reported separately (skippedRenderUnsupported vs
    skippedUnsupportedVariants) instead of being conflated.

Regression tests cover both halves: that a partial failure is recorded rather
than ignored, and that the gate skips recorded variants while still flagging a
genuine gap. Full suite 2480 pass / 0 fail, typecheck + lint clean.