…tic keyless signing

Extract the Sigstore Fulcio + Rekor keyless-signing plumbing from
KeylessAttester.Attest into a reusable SignStatement primitive that
takes pre-built in-toto Statement bytes. The Attest method keeps its
existing public contract — it now builds the SLSA Provenance v1
Statement (caller-specific) and delegates the DSSE wrap, ephemeral
keypair, Fulcio cert issuance, Rekor entry, and Sigstore-bundle
marshal (caller-agnostic) to the new function.

Motivation:

The recipe-test-attestation pipeline landing under NVIDIA#754 needs to sign
an in-toto Statement carrying a different predicate type
(https://aicr.nvidia.com/recipe-evidence/v1) but the same DSSE +
Fulcio + Rekor + protobuf-JSON output shape. Without this split,
that pipeline would duplicate ~80 lines of identical signing
plumbing in pkg/evidence/attestation/signer.go. Extracting now means
the second caller is a thin client of the bundler's primitive.

API surface:

  - SignOptions{OIDCToken, FulcioURL, RekorURL}
  - SignedAttestation{BundleJSON, Identity, Issuer, RekorLogIndex}
  - SignStatement(ctx, statementJSON []byte, opts SignOptions) (*SignedAttestation, error)

The new return type also exposes the Fulcio cert's OIDC issuer claim
and the Rekor inclusion-proof log index — a strict superset of the
existing identity-only extraction. Callers that only need identity
can ignore the extras; pkg/evidence/attestation will consume all
three to populate its pointer file's signer block.

Internal cleanup:

  - extractSignerClaims (identity + issuer) replaces extractSignerIdentity
    as the canonical extractor; extractSignerIdentity is retained as a
    thin wrapper so the existing keyless_test.go assertions stand
    unchanged.
  - extractIssuerExtension parses the Fulcio OIDC-issuer X.509
    extension (OIDs 1.3.6.1.4.1.57264.1.1 legacy and .1.8 current).
  - extractRekorLogIndex returns the first transparency-log entry's
    index, or 0 when no Rekor entry was created.

Tests cover input validation, identity-from-email and identity-from-URI
extraction (matching the two Fulcio OIDC modes), issuer extension
parsing, Rekor log-index extraction, and the empty-bundle / nil
fallbacks. The signing call itself still needs a live Sigstore stack
and remains exercised only via integration tests.

Related: NVIDIA#754, ADR-007 (`docs/design/007-recipe-evidence.md`)

extractIssuerExtension treated both the legacy (OID 1.3.6.1.4.1.57264.1.1)
and current (OID 1.3.6.1.4.1.57264.1.8) Fulcio issuer extensions as raw
UTF-8 bytes. The current OID is actually a DER-encoded ASN.1 UTF8String
(per the Sigstore Fulcio OID spec), so the returned issuer string carried
the leading ASN.1 tag/length prefix and could not be compared against the
expected issuer URL.

Decode the current-OID value via asn1.Unmarshal; keep raw-bytes handling
for the legacy OID where the unwrapped form is correct. Consolidate the
three TestExtractRekorLogIndex_* funcs into one table-driven test, and
update the test cert helpers to stamp the issuer using a shared helper
that DER-encodes the value the same way Fulcio does.

Two cleanups in pkg/bundler/attestation.SignStatement and one test
addition:

  - Wrap the sign.Bundle call in context.WithTimeout against a new
    defaults.SigstoreSignTimeout (2 minutes) so a hung Fulcio or
    Rekor peer cannot block the CLI indefinitely. The shorter caller
    deadline still wins if it is already attached.

  - Demote the post-sign INFO line so it carries only the public
    rekorLogIndex; the Fulcio SAN (identity) and OIDC issuer URL are
    moved to DEBUG. Identity is user PII for interactive OIDC and
    shouldn't show up in default-level CLI output; callers that need
    it for auditing still receive both via SignedAttestation.

  - Add TestExtractIssuerExtension covering both Fulcio OIDs, a
    malformed-ASN.1 value, and the no-issuer-extension case.

… precedence

Three cleanups in pkg/bundler/attestation:

* Remove the extractSignerIdentity wrapper. It was a one-line shim
  around extractSignerClaims kept for the older tests; migrate the
  tests to call extractSignerClaims directly so the package has one
  parsing surface instead of two.

* Demote the post-sign INFO line in KeylessAttester.Attest so the
  Fulcio SAN identity flows to Debug only. Mirrors the SignStatement
  contract — identity is user PII for interactive OIDC. Callers that
  need the value still read it back via KeylessAttester.Identity().

* Rework extractIssuerExtension from a single-pass switch to a
  two-pass scan that prefers the current OID (1.3.6.1.4.1.57264.1.8)
  over the legacy one (1.3.6.1.4.1.57264.1.1) regardless of X.509
  extension ordering. A single-pass switch makes the result
  order-dependent for transitional certs that carry both. Add two
  table entries pinning the ordering invariant in both directions.

…te issuer extensions

Two correctness fixes in pkg/bundler/attestation, plus a tidy-up:

* SignStatement now distinguishes context.DeadlineExceeded from a
  generic Sigstore failure. A SigstoreSignTimeout-bounded deadline
  surfaces as ErrCodeTimeout (HTTP 504 at the API boundary, telling
  the caller "the signing flow took too long"), while Fulcio refusals
  and Rekor 5xx still flow through as ErrCodeUnavailable (502).

* extractIssuerExtension now inspects asn1.Unmarshal's rest return
  and rejects any extension carrying trailing bytes after a
  well-formed UTF8String. A tag-stuffed value that decodes cleanly
  for the first segment but appends extra data could otherwise be
  honored silently, masking either truncation or an attempt to
  smuggle additional content. Adds a table case pinning the
  rejection.

* Consolidate the seven TestExtractSignerClaims_* identity tests in
  keyless_test.go into one table-driven TestExtractSignerClaims_Identity
  covering email/URI SAN, single Certificate vs X509CertificateChain,
  nil/missing/malformed cert, empty chain, and no-SAN cases. Pure
  refactor — same coverage.