FABRIC.md item 4.4ac: scrollback during boot/POST, not just from REPL

vt100.c: font_8x16/bitmap-mode boot output previously had no scrollback
at all -- g_shadow/g_ring were only allocated in vt100_enable_ttf(), so
POST/self-test/heartbeat text was gone the instant it scrolled off,
recoverable only from the serial log. Gives boot mode its own ring/
shadow pair (bitmap cell geometry, 4096-line capacity), frozen as a
snapshot the moment vt100_enable_ttf() switches to the TTF-geometry
pair, per the two-independent-rings design scoped with Captain Bob.
scrollback_line_at()/scrollback_redraw()/vt100_scroll_back() now walk
all four segments (boot ring, boot shadow, TTF ring, TTF shadow) as one
continuous history, so PgUp from the REPL reaches back through POST.

Three-arch QEMU boot + logs clean (amd64/aarch64/riscv64, no faults, no
dictionary/parity regressions). Visual verification that PgUp actually
recalls POST text still needs an interactive GTK screendump -- noted as
open in FABRIC.md, same pattern as 4.4ab's screendump.

Also includes BLOCK_MAP.md/artemis.img regenerated by these builds, and
the acceptance-boot logs (plus stray logs from an earlier QEMU-instance
collision during testing -- kept per repo convention, logs are audit
artifacts, not deleted).

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

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