Rename FABRIC series: FABRIC.md->0, FABRIC-2.md->1, FABRIC-3.md->2, FABRIC-4.md unchanged

FABRIC.md -> FABRIC-0.md
FABRIC-2.md -> FABRIC-1.md
FABRIC-3.md -> FABRIC-2.md (the current/living document)
FABRIC-4.md unchanged (new #3 to follow separately)

Every cross-reference repo-wide updated to match, including doc-comment
citations inside kernel source (.c/.h) files -- done via an ordered
placeholder substitution (FABRIC-3.md->placeholder2, FABRIC-2.md->
placeholder1, FABRIC.md->placeholder0, then placeholders resolved to
final names) in a single pass per file to avoid double-shifting
already-renamed references.

One line in capsules/font.4th grew past the 64-char block-format limit
as a side effect of the longer filename; shortened it and reverified
with mkcapsule --lint (34/34 pass) before rebuilding.

Verified 3-arch boot to ok> (amd64/aarch64/riscv64, each in the
foreground) after the fix; logs and DoE CSVs from this session's
verification runs included per this repo's own audit-artifact
convention.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019YcT3H2PQeyujrzjqS3Var

src/starkernel/arch/riscv64/arch.c

@@ -69,7 +69,7 @@ void arch_early_init(void)
      * activated here, per its own load_cr3() no-op outside __x86_64__) and
      * has no present use for virtual memory on this ISA, so there is no
      * reason to inherit firmware's Sv57 mapping -- which is confirmed to
-     * have at least one hole (PLIC_THRESHOLD, FABRIC.md item 4.3.5a).
+     * have at least one hole (PLIC_THRESHOLD, FABRIC-0.md item 4.3.5a).
      * ExitBootServices() has already completed several checkpoints before
      * this function runs (ConOut/GOP done, BootServices exited per the
      * "[CKPT 008]" trace), so nothing downstream depends on firmware's