nmc(P1f): HeaderChain LevelDB persistence (h/tip/height) + 6 KATs (52->58)

[frstrtr]

P1f LevelDB persistence leg (NMC embedded merge-mined chain)

Stacks on the P1f get_locator slice conceptually but is independent — touches
disjoint regions of header_chain.hpp, so it bases cleanly on master (does not
require #200/get_locator to land first).

What it does

• Write path: every accepted header is persisted on the connect_locked
  success path under "h"+block_hash(32) → serialized IndexEntryDiskV1, plus
  "tip"(32B) and "height"(4B BE) pointers, in one commit_sync() batch.
  Rejected (idempotent / orphan / activation-gate) headers write nothing.
• Read path: init() opens the store (when db_path given) and
  load_from_db() rebuilds m_index from the "h" prefix, then restores
  m_tip / m_tip_height / m_best_work from the tip entry.
• Degrades gracefully: empty db_path or open failure → pure in-memory, no crash.

Mirror-of-btc with a documented divergence

Adapted from btc::coin::HeaderChain persistence. No "i" height-index key:
NMC keeps a full-residency m_index (not btc's bounded LRU + separate height
index), so the "i" schema has no consumer here. The "height" key collision
with the "h" prefix scan is handled by a size()==33 guard (covered by a KAT).

P1f-DEFER

AuxPow blobs are not serialized yet — only a has_auxpow presence flag. A
reloaded merge-mined header keeps its HEADER_VALID_CHAIN status; its auxpow
comes back nullopt. The blob lands behind this same flag in the next sub-leg
(nmc::coin::AuxPow has no serialization yet; not hand-rolled here per the fence).

Tests / fence

+6 KATs (NmcP1fPersist): disk-format round-trips (incl. status + aux-flag) and
real LevelDBStore reopen round-trips under a temp dir. Suite 52 → 58 green,
nmc_auxpow_merkle_test EXIT=0 locally. NMC-tree only (two files:
coin/header_chain.hpp, test/auxpow_merkle_test.cpp); rides the allowlisted
test target — no build.yml / CI change. GPG-signed, attribution-clean.