feat(keyring): switch deriveContextHash to per-account-xpub semantics

[jrwbabylonlab]

Summary

Changes deriveContextHash IKM source from a fixed BIP-32 path
(wallet-level) to the connected leaf's private key — the BIP-32 leaf
at the active receive-address path (e.g. m/44'/0'/0'/0/0). HD and
imported paths are now symmetric: one connected public key = one IKM =
one output.

Behavior change

Scenario	Before	After
Same wallet, different receive addresses	Same	Different
Same wallet, account 0 vs account 1	Same	Different
Same wallet, BIP-44 vs BIP-86	Same	Different
Same connected leaf called twice	Same	Same
Imported wallets	Per-key	Per-key (unchanged)

dApps that need cross-leaf shared state (e.g. account-scoped data) must
do that aggregation themselves — they're connecting one address at a
time, so they get exactly one output per connection.

Breaking change is sanctioned by @experimental

deriveContextHash is marked Experimental in the inpage method,
type registry, API docs, doc index, and release notes. The API docs
explicitly tell integrators to use feature detection. Salt unchanged —
the IKM-source change alone produces independent outputs from v1.0.

Implementation note

The HD-keyring lookup matches the connected pubkey against the keyring's
existing leaf wallets and uses that leaf's private key directly as IKM.
No new BIP-32 derivation path; reuses whatever path the keyring is
already authorized for.

Spec updates: babylonlabs-io/babylon-toolkit#1540