test(corona): argument-validation tests for Bootstrap/Reshare/Reanchor + alias surface

[abhicris]

protocols/corona/ shipped without a dedicated _test.go file. The alias surface (Bootstrap, Reshare, Reanchor, ShareForParty, GenerateKeys, NewParams, validatorIDs) does real argument validation before delegating to the luxfi/corona kernel, but none of those guards were pinned by tests. Corona is the R-LWE primitive in the Polaris triple-PQ profile (BLS ‖ Pulsar ‖ Corona ‖ Magnetar) — the alias surface here is what luxfi/consensus imports, so its validation contracts are part of the chain-facing API.

What this PR adds

protocols/corona/corona_test.go — 17 tests, all PASS in 0.018s.

Bootstrap (4)

• empty validators → ErrEmptyValidators
• t=0 → ErrInvalidThreshold
• t > n → ErrInvalidThreshold
• nil entropy does not panic (falls back to crypto/rand.Reader)

Reshare (3)

• nil KeyEra → error mentioning "nil key era"
• nil-era + nil validators (guard-order check)
• nil-era + threshold > n=1

Reanchor (3)

• empty validators → ErrEmptyValidators
• t=0 → ErrInvalidThreshold
• t > n → ErrInvalidThreshold

ShareForParty (3)

• nil state → error mentioning nil-share-state
• party not in set → ErrPartyNotInSet (error mentions the missing id)
• happy path returns the exact same *KeyShare pointer

Alias surface (2)

• NewParams returns non-nil *Params with no error
• GenerateKeys (2-of-3 trusted-dealer) returns 3 non-nil shares + non-nil *GroupKey from a deterministic seed

validatorIDs unexported helper (2)

• preserves order and length for non-empty input
• returns non-nil empty slice for nil input

Verification

$ go test ./protocols/corona/ -count=1 -v -timeout 120s
PASS — 17/17 in 0.018s

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

Notes

• Tests live in package corona (not corona_test) so the unexported validatorIDs helper can be reached. The two failing-case tests for Reshare use nil KeyEra instead of constructing a synthetic one — building a real *KeyEra for failure paths is out of scope; the nil-era guard covers the validation surface for those branches.
• The Bootstrap_NilEntropyDefaultsToCryptoRand test logs (not fails) when the kernel returns an unrelated error in the constrained CI env — the contract being pinned is no panic on nil entropy.