test(pulsar): alias-surface tests for ParamsFor + Sign/VerifyCtx round-trip

[abhicris]

protocols/pulsar/ had no _test.go even though the surface is what luxfi/consensus imports (per the alias-layering comment at the top of pulsar.go). The sibling protocols/corona/ package was in the same shape until PR #22 (this session); this PR brings pulsar up to parity.

What this PR adds

protocols/pulsar/pulsar_test.go — 13 tests (+3 subtests), all PASS in 0.013s.

ParamsFor / MustParamsFor (4 tests / 3 subtests)

• ParamsFor(ModeUnspecified) → error, nil params
• ParamsFor(P44 / P65 / P87) → returns the pre-built ParamsXX *Params singleton by pointer equality (the cache-keying contract documented in pulsar.go)
• MustParamsFor(ModeUnspecified) panics
• MustParamsFor(ModeP65) returns the ParamsP65 singleton

GenerateIdentity / NewIdentityDirectory (3 tests)

• GenerateIdentity(rand.Reader) returns non-nil *IdentityKey
• Two generations from equivalent deterministic seeds both succeed
• NewIdentityDirectory({}) does not panic on empty input (alias contract)

Single-party FIPS 204 round-trip (5 tests)

• GenerateKey(ParamsP65, rand.Reader) returns non-nil *PrivateKey
• Sign + VerifyCtx happy path
• VerifyCtx rejects a tampered message
• VerifyCtx rejects a wrong context and still verifies the right one

The VerifyCtx tests use a small test-local helper publicKeyFromPrivate that calls sk.Public() through a runtime interface assertion (type publicer interface { Public() *PublicKey }), so the test stays stable if the kernel reshapes the PrivateKey API.

Verification

$ go test ./protocols/pulsar/ -count=1 -v -timeout 120s
PASS — 13/13 in 0.013s

No changes to non-test files. No regression in the existing suite (there was no pulsar test coverage to regress).

Notes

• This is the alias-surface contract test, not a substitute for kernel-level KATs (those live in luxfi/pulsar/ref/go/pkg/pulsar). It pins what the consensus layer relies on through the alias path.
• Pairs naturally with PR test(corona): argument-validation tests for Bootstrap/Reshare/Reanchor + alias surface #22 (corona alias tests, same session).