feat(license): replace forgeable rotor cipher with Ed25519-signed tokens

[krisarmstrong]

Summary

Replaces Stem's forgeable license scheme with offline-verifiable Ed25519-signed tokens, harmonized with niac (#802) and the keygen tool. The old 16-char rotor-cipher + self-checksum shipped its own generator (GenerateLicenseKey) inside every binary — any copy could mint valid Professional keys. This is the Ed25519 license item from the stem/niac remediation plan (ADR-0007).

Token format (signing.go)

MSN1.<base64url(payload)>.<base64url(signature)>

• payload = canonical JSON {v, product, code, serial, tier, maxDevices, iat, exp}
• signature = 64-byte Ed25519 over the exact payload bytes
• binary embeds only the public key; private key lives solely in the keygen tool

What changed

• Verifier.Validate checks the signature before interpreting any field, then enforces scheme/version/product==stem/tier/code-pairing (1001→Reflector, 2001→Professional, 3001→Enterprise)/expiry. Tier→feature mapping stays in-binary; deprecated Enterprise tokens keep validating and grant Pro features. TierProfessional/TierTestSuite alias and wire codes preserved.
• Deleted cipher.go + GenerateLicenseKey (the forgeability vector).
• Manager gains a verifier field + NewManagerWithVerifier so tests activate ephemeral-key tokens without the production private key.
• Tests rewired: rotor/checksum/alphanumeric tests removed (tested deleted code); GenerateLicenseKey→signTestKey, ValidateLicenseKey→testVerifier, NewManager→NewManagerWithVerifier. Added forge/tamper rejection + per-tier production-signed contract vectors.
• FormatKey no longer strips characters (base64url uses -/_).

Security property

TestForgeryRejected/TestForgeryRejectedInternal mint an attacker-key token and assert the production verifier rejects it with ErrLicenseInvalid. Validation stays fully offline (no phone-home).

Pre-launch: no issued keys to honor. Embedded key is a pre-launch key — rotate via keygen before GA (regenerate key + contract vectors together).

Verification

• go build ./..., go vet ./... clean
• golangci-lint — 0 issues (license + consumers); one narrow documented unparam exclusion for the signTestKey (string,error) drop-in shape
• go test -race ./internal/license/ (ok 101s) + ./internal/api/ (ok 512s)

[github-actions]

License Compliance Report

All dependencies pass license compliance checks

Go Dependencies

• Unknown: 31 package(s)
• MIT: 26 package(s)
• BSD-3-Clause: 16 package(s)
• Apache-2.0: 11 package(s)
• BSD-2-Clause: 1 package(s)

npm Dependencies

See full report in workflow artifacts

Allowed Licenses: MIT, Apache-2.0, BSD-*, ISC, CC0-1.0, MPL-2.0
Forbidden: GPL, AGPL, SSPL (strong copyleft)