user-font-demo.4th: prove the override mechanism (item 4.3.6e)

Punch list §25 item 4.3.6e complete. Optional capsule, not wired into
the boot chain, overriding DISPATCH-UPPER's codepoint 65 (G-A) with a
marked alternate glyph while every other letter falls through to
font.4th's originals unchanged.

Corrects §27.6.5: redefining the DISPATCH-* bucket word alone is not
enough, since DISPATCH-GLYPH/DRAW-GLYPH are themselves compiled early
against font.4th's own dispatch words (same early-binding finding as
4.3.6c, one layer up). Verified both ways: the partial override (bucket
word only) left DRAW-GLYPH still drawing the original glyph; redefining
DISPATCH-GLYPH/DRAW-GLYPH too made the override take effect.

Three-arch acceptance boot clean, Stadium conservation unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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