Add provisional BIP44 coin type metadata

[mtg0001]

Summary

• Reserve Bitflash's provisional BIP44 coin type as 4346950, with the intended SLIP-0044 registry row 4346950 | BITFLASH | Bitflash until an official entry exists.
• Persist hdcointype alongside the HD wallet schema metadata, while inferring old seeded wallets to the provisional value in memory.
• Add BIP44 path helpers and route the existing legacy m/index' derivation through the shared BIP32 path engine without changing wallet behavior.
• Surface the coin type in recovery audit / Wallet Safety and cover it in wallet HD self-tests.

Why

SLIP-0044 currently has FLASH assigned to Flashcoin and BTF assigned to Bitcoin Faith, and no Bitflash entry. Reusing either would be worse than being explicit. This PR uses BITFLASH as the proposed registry symbol and keeps the value provisional and stored as metadata so preview wallets remain recoverable even if an official coin type is assigned later.

This is still preparation only: the wallet continues to use the legacy schema and this build still refuses to operate a BIP44-schema wallet until the actual BIP44 derivation/restore PR lands.

Stack

Stacked after #121 and #122.

Testing

• git diff --check
• mingw32-make -f makefile.mingw tests from MSYS2 UCRT64 in src/

[mtg0001]

Manual smoke test on the PR branch, without merging and with a disposable datadir:

• Started from the stacked branch containing v1.2.15 + Add BIP32 path derivation foundation #121 + Add HD wallet schema metadata #122 + this PR.
• Ran /recoveryaudit on a fresh datadir: no phrase installed, zero balance.
• Ran /newphrase: phrase installed successfully.
• Restarted through a new CLI invocation and ran /recoveryaudit again:
  • recovery phrase: present
  • derivation schema: legacy-hd
  • BIP44 coin type: 4346950 (provisional BITFLASH)
  • derived keys known to this wallet: 101
  • receive/change counters: 0/0
• Ran /showderived=3: derived the first three addresses successfully.

Automated checks also passed:

• git diff --check
• mingw32-make -f makefile.mingw tests from MSYS2 UCRT64 in src/

This does not claim full BIP44 wallet behavior yet. It only verifies this preparation PR: metadata persistence, audit visibility, path helper coverage, and unchanged legacy derivation behavior.

[Bitflash-sh]

Landed in #124, which contains this branch — the four were stacked, so squashing the top brought all of them into main as 876d7a7. Closing this one as merged rather than leaving it open against changes that are already in.

The real-coin verification is written up on #124.