v0.41.1 — a failed keychain read must not mint a new wallet

BLOCKRUN_KEYCHAIN=strict could mint a new wallet and orphan your funded
one. 0.41.0 shipped keychainLoad() returning null for two different
facts: the keychain has no entry, and the keychain could not be read. Under
strict the plaintext file is already gone, so the caller took a transient
read failure — a locked keychain, an ACL denial, a timeout — as "no wallet
here" and fell through to getOrCreateWallet(), which does what its name says.
The funded wallet was still sitting in the keychain; the MCP just started
signing with a fresh empty one. keychain.ts names this exact scenario as the
worst possible failure in a comment at the read site, and then nothing acted
on it.

Reads now return a discriminated found / absent / error. Only absent
may create a wallet. An error with no file left stops with an actionable
message naming the keychain fault and pointing at BLOCKRUN_WALLET_KEY — a
loud failure is recoverable, a silently substituted wallet is not. auto mode
was never exposed: the file it keeps is found first.

The music poll deadline is now real code rather than an assertion about
constants. The 0.40.x guard asserted auth - margin < submit + budget + timeout + auth, which contains auth on both sides and is therefore true for
every input — it verified nothing. The deadline calculation moved into
pollDeadline() and the test now exercises it: the poll budget binds at
today's constants, the authorization takes over above the 445s crossover, and a
poll attempted at the deadline is refused rather than issued.