Implements WebAuthn L3 §10.1.5 write and a libwebauthn-side delete
on top of CTAP 2.2 §6.10.6 update-or-erase. The platform fetches the
existing large-blob array, drops any entry that decrypts under the
credential's largeBlobKey, optionally appends a fresh encrypted entry,
and re-uploads the canonical CBOR array with a fresh SHA-256 trailer.
Foreign entries (different key, malformed, unknown fields) are
preserved byte-for-byte per the §6.10.2 platform contract.

Upload is chunked per maxFragmentLength with pinUvAuthParam computed
as authenticate(token, 32*0xff || 0x0c 0x00 || u32_le(offset)
|| SHA-256(set)) per §6.10.2. The Ctap2GetAssertionRequest now ORs
LARGE_BLOB_WRITE into its permissions when write or delete is
requested so user_verification negotiates a token covering the lbw
permission.

To keep PIN-protected devices working under UV=Discouraged, the
Ctap2UserVerifiableRequest trait gains needs_pin_uv_auth_token,
which suppresses the OnlyForSharedSecret downgrade in
user_verification. Unprotected authenticators continue to accept the
write without auth params per spec line 137.

Delete with no matching entry returns LargeBlobError::NoMatch
(written=false), matching the strict §6.10.6 'Return an error' branch
at line 303.

Exercises the WebAuthn write/delete extensions against the virt
authenticator end-to-end:
- write then read returns the planted bytes
- second write replaces the first entry
- delete after write removes the entry; subsequent read returns no blob
- delete with no prior entry reports written=false

Preserve foreign largeBlobArray entries by their exact bytes on
read-modify-write rather than re-serialising them, which also drops a
redundant re-encode of the new entry. Treat a valid-trailer but
unparseable array as a fail-safe error rather than clobbering it. Satisfy
the production clippy::indexing_slicing denies in the write path. Refuse
the U2F downgrade for largeBlob write and delete.

Adds tests for nonce uniqueness across writes, the pinUvAuthParam
protocol-1 path, the JSON write input, the PIN-protected write token
ceremony under UV=Discouraged, foreign-blob survival across write and
delete, the single-allowCredential guard, and shrink-on-replace.