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/heartbeat.c

@@ -15,7 +15,7 @@
  * timer.c contributes only heartbeat_read_counter() — the one thing that is
  * genuinely per-ISA (rdtsc / rdtime / CNTPCT_EL0).
  *
- * Top half / bottom half split (FABRIC.md §25.1 item 0.8, per the GAP-A1
+ * Top half / bottom half split (FABRIC-0.md §25.1 item 0.8, per the GAP-A1
  * ruling in §16.4/§18.4): heartbeat_tick() is called from interrupt context
  * and does nothing but read the counter, bump TIME-TICKS, and latch a
  * pending sample. heartbeat_service() runs on the mainline (the REPL idle
@@ -23,7 +23,7 @@
  * state — the engine stays on the virtual tick per §18.4, unchanged by this
  * file.
  *
- * Adaptive re-arm period (FABRIC.md §26, ruled 2026-08-03): Loop #7
+ * Adaptive re-arm period (FABRIC-0.md §26, ruled 2026-08-03): Loop #7
  * (vm_runtime.c) computes an execution-derived stable/volatile signal and
  * calls heartbeat_set_adaptive_period_ns() with it, rescaled to this file's
  * kernel-appropriate base (10 ms, matching the 100 Hz rate item 0.1-0.7
@@ -47,7 +47,7 @@
  * rate established throughout items 0.1-0.7. Deliberately NOT
  * HEARTBEAT_TICK_NS (include/starforth_config.h) -- that constant is the
  * hosted pthread-worker's 10 µs base and is three orders of magnitude too
- * fast for a bare-metal ISR period (FABRIC.md §26.3). Loop #7's decision
+ * fast for a bare-metal ISR period (FABRIC-0.md §26.3). Loop #7's decision
  * logic is reused unmodified; only the base it scales differs. */
 #define HEARTBEAT_BASE_PERIOD_NS  10000000ULL
 
@@ -213,7 +213,7 @@ const TimeTrustState *heartbeat_state(void)
 }
 
 /**
- * @brief Set the adaptive re-arm period (FABRIC.md §26).
+ * @brief Set the adaptive re-arm period (FABRIC-0.md §26).
  *
  * Called from vm_runtime.c's Loop #7 site on the mainline execution path
  * (never interrupt context) with a value already rescaled to this file's