platformer: sprite-based status bar with score + lives

Upgrades the platformer's "live coin count" into a proper heads-up
display that stays pinned to the top of the viewport while the
nametable scrolls. Left side: coin icon + two-digit stomp tally.
Right side: red heart icon + single-digit lives counter. Both ride
through the GameOver screen without jumping position, so the death
banner reads as a continuation of the same run.

Wire-up: three new cross-state bits — score now accumulates across
lives, `lives` starts at 3 and decrements in `GameOver.on_enter`,
and the GameOver → Playing retry bounces to Title instead when the
last heart is spent (Title's `on_enter` refills both).

Tile pipeline: ten decimal digits + a heart glyph added to the
committed Tileset (generator source in `scripts/gen_platformer_tiles.rs`
kept in sync). Digits use `c` (white) so they read against the
sky; the heart uses `a` (red) to match the cap/brick palette.

Division workaround: the obvious `stomp_count / 10` / `% 10` pair
miscompiles near state transitions — the built ROM cycles
Title → Playing → Title once per blink period with Playing
surviving exactly one frame. Swapping both calls for repeated
`while r >= 10 { r -= 10 }` helpers fixes it. Documented as a
new entry in `docs/future-work.md` so the next person reaching
for `/` or `%` knows to check there first.

Goldens, docs/platformer.gif, and the top-level + examples README
entries all refreshed in the same commit.

tests/emulator/goldens/platformer.audio.hash

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