fix(symbols): flag catalog entries the build macOS cannot render

SF Symbols 8.2 (auto-downloaded since its release today) catalogs
macOS-27-era names like private/f1; CoreGlyphs on older macOS has no
glyph for them, every prerender variant fails, and the snapshot
completeness gate hard-failed the CI stable build (run 27300445215,
27 missing variants — all of f1's weight x scale matrix). My macOS-27
beta build rendered it fine: the failure class is strictly
catalog-newer-than-OS.

Mirrors the v18 bitmap_only precedent: v27 adds
sf_symbols.render_unsupported; the prerender pass flags symbols whose
EVERY variant failed (partial failures stay loud and still fail the
gate) with one summary warning; the validator skips flagged rows; and
renderSfSymbol answers with the availability story — the glyph ships
with a newer macOS, beta snapshots built there carry it.