sharechain/v37: consensus fixes + Merkle lane digest + messaging reservations

[frstrtr]

Lands the post-base-library V37 authoring work onto v37-dev (follows PR #84 which landed the base library). Folds 8 authoring-branch commits; source history retained on the authoring branch (tip 7a416aa).

Scope (src/sharechain/v37/ only — 6 files, +744/-70)

• Consensus fix: lane digest keyed by canonical identity, not intern ids
• Merkle lane digest with inclusion proofs (tamper/index/stale-root rejection tests)
• Reassessment: inv-decay headroom guard, rewind fold undo
• Review fixes: rewind/rebuild boundary, descriptor binding, geometry guards
• Messaging reservations: V36 authority share messaging at L0; reserve L0F_MINER_MSG
• Notes: OQ-10 (V36 authority keys carry forward), OQ-11 (fold-bounded message visibility)
• Spec errata folded: E-1 Q62, E-2 wider comp/sum storage, E-3 lane-geometry validity, E-4 canonical-identity digest order, E-5 journal does not cross rebuilds

Verification

• Standalone (header-only, stdlib-only): `g++ -std=c++20 -O2 src/sharechain/v37/test/v37_test.cpp` → 100509 checks, 0 failures; reproduced on the workstation. Also clean under ASan/UBSan on the authoring host.
• Consensus gate: independent full-scan reference matches incremental accumulators bit-exact across two geometries, through epoch rebuilds/folds/evictions.

Integration follow-ups (tracked in IMPLEMENTATION-NOTES.md — NOT in this PR)

1. Donation adapter (consensus decision — operator): V37 has no V36 65535-donation equivalent; unresolved = donation outputs silently unpaid post-migration.
2. att(uint288)->w_raw(u64) clamp assert; 3. multi-head Lane + rebuild-from-tracker API at integration; 4. full in-tree CI build (test target gated on BUILD_TESTING); 5. agreement tests before any core/ helper swap; 6. messaging wiring (ENVELOPE_MINER in share_check, view-layer index).

ci-steward / btc-heap-opt: full CI build verification. Merge held for operator.