starkernel: retarget REPL glyph rendering to TTF-TEXT's rasterizer (4.4j)

font_8x16.c keeps rendering everything through and including POST;
TTF-TEXT's rasterizer takes over at the interactive REPL boundary
(sk_repl()) via a new runtime mode switch, vt100_enable_ttf()
(console_fb_enable_ttf() wrapper), not a compile-time swap -- both
backends coexist in the same binary since boot/POST must stay
font_8x16.c per this item's own done-when.

TTF-TEXT (the FORTH word) isn't directly callable from vt100.c -- VM
stack arguments, different call shape than a one-glyph cell draw. Used
hal/ttf.c's VM-independent primitives directly instead (same rasterizer
TTF-TEXT itself calls underneath), added as a native C helper in
vt100.c. Lazily loads fonts:JetBrainsMono-Regular.ttf and kmallocs a
96-slot raster cache (covers all 95 printable ASCII, no eviction
thrash) on first switch.

Cell geometry changes at the switch (mode-aware cell_w()/cell_h()):
provisional 12x24 TTF cell (600/1000em * 20px = 12px exactly, using
4.4i's confirmed-uniform hmtx advance width) vs font_8x16's fixed 8x16
-- cols/rows re-derived and screen cleared at the switch point, same as
vt100_init() itself does. Final REPL text size is 4.4m's decision, not
this item's.

Also fixes the second call site 4.4i flagged: erase_line_range() now
uses one fb_fill_rect() instead of a per-cell font_8x16-specific blank
glyph draw, consistent with erase_display(2)'s full-screen case.

Three-arch verified: amd64/aarch64/riscv64 all reach ok>, POST
Failed: 0, identical dict-hashes. amd64 screendump shows real
proportional JetBrains Mono letterforms on the REPL tail, visibly
distinct from every prior font_8x16 screenshot.

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

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