fix: don't evict LAContext on user-cancelled Touch ID prompts

[jgowdy-godaddy]

Problem

Users experiencing repeated Touch ID prompts and eventual password fallback after cancelling Touch ID 3+ times. The git pull error showed:

sign_and_send_pubkey: signing failed for ECDSA "..." from agent: agent refused operation

Agent logs showed:

• SE_ERR_SIGN (code 3): signing failed
• SE_ERR_KEYCHAIN_LOAD (code 10): keychain load failed

Both errors triggered LAContext eviction, forcing fresh Touch ID prompts on every retry even though the LAContext was still valid.

Root Cause

Generic Swift error codes (3, 10) collapsed all failures into a single code, losing the actual OS error details (LAError.userCancel, errSecUserCanceled). Rust couldn't distinguish "user cancelled" from "real error" and evicted the LAContext on both.

Failure cycle:

1. Sign fails (user cancels Touch ID)
2. LAContext evicted
3. Next sign creates fresh LAContext → fresh Touch ID prompt
4. User cancels again → evicted again
5. After 3+ cancellations, macOS falls back to password

Solution

• Add SE_ERR_USER_CANCEL (code 16) for explicit user cancellation
• Map LAError.userCancel and errSecUserCanceled to the new code
• Add Error::UserCancelled variant to Rust
• Update should_evict_lacontext to NOT evict on UserCancelled
• Add error logging to Swift bridge catch blocks for diagnostics

Result

When a user cancels Touch ID:

• LAContext stays cached
• Next sign within cache TTL reuses same LAContext (no prompt spam)
• Agent logs show actual error: "enclaveapp: se_sign: user cancelled (LAError -2)"

Testing

• All existing tests pass
• Added test for Error::UserCancelled display message