FABRIC-3.md §I.2: FIRSTTOUCH + migration state machine (blk_meta_relocate_devblock)

Closes the block-subsystem punch-list item -- built exactly to §F.11's
already-decided algorithm after re-verifying it against current
blk_meta_t (a 2026-09-03 re-scoping note had wrongly claimed the chain
fields no longer existed; they do, untouched by BMAPFMT).

blk_firsttouch_claim(): one linear scan of Artemis's own device
(new blk_get_first_disk_range(), correctly bounding the scan instead of
the global multi-device LBN space), scattered-chain claim via
prev_block/next_block/chain_length, owner_fp stamped on every member
devblock, fails outright with no partial claim.

blk_meta_relocate_devblock(): the real migration primitive -- bridges
the existing FORTH-block-granularity blk_subsys_relocate_block() up to
devblock granularity (BLK_PACK_RATIO=3, corrected mid-design), running
it 3x and transferring blk_meta_t ownership fields. The "migration
state machine" turned out to be just the 2 states BLK_FLAG_MIGRATING
already reserved; the real design work was the trigger. Two were
scoped in conversation (overflow onto Artemis; heat-based wear
leveling); heat/wear-leveling is built and wired into sk_repl_idle()
via blk_meta_t.write_count. Overflow is deliberately left open,
precisely scoped (needs a slot-lookup-by-device-pointer call site
threaded from WIREBIND) rather than guessed at.

Also flagged, not fixed: BMAPFMT's owner_fp/CLAIMED and the pre-existing
BAM allocator are two parallel, unreconciled accounting systems --
FIRSTTOUCH/relocate only touch the former.

Verified 3-arch boot to ok> (amd64/aarch64/riscv64, each in the
foreground); 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

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