term: 3 more renderer wins - 10.9x scroll, 2.7x putc vs pre-cache

1. color_setup memoization: a call with the same (colL,colR) pair as
   last time returns immediately (masks/attr still valid in WRAM).
   Runs of same-colored cells - i.e. almost all text - hit this.
2. build_tile blank fast path: two spaces = bg-only planes, 8 constant
   rows; term_scroll's cleared line and every blank region skip the
   glyph pipeline entirely.
3. build_tile two-phase rewrite: combine both glyphs into wRowBuf with
   pointers in registers (the old per-row 16-bit WRAM pointer walk was
   most of the blit), then compose planes unrolled with plane-0 masks
   in B/C.
4. term_write_tilemap attr pass: split each row at the pos-256 VRAM
   bank boundary into two tight cache->tilemap copy runs - no per-tile
   addressing or bank test.

Measured (emulator cycle counter, ANSI test screen):
  term_scroll         857k -> 248k (attr cache) -> 78.5k  T-cycles
  term_write_tilemap  723k -> 114k              -> 45.4k
  term_putc           18.2k                     -> 6.7k
A scroll is now ~1.1 frames of guest CPU (was 12); a full 40-char line
prints in ~63ms of guest time (was ~173ms).

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