sercon v0.5.2

Third Moderate cut. api.jwt.* — sign / view / validate over
HMAC-signed JWTs. New dependency:
github.com/golang-jwt/jwt/v5 v5.3.1 (pure Go, de facto standard).
Asymmetric algorithms (RSA / ECDSA / EdDSA) are deliberately split
off into a follow-up cut so the JS-side key-shape design can be
done in isolation.

Added

• api.jwt.sign(claims, secret, opts?) produces a
  compact-serialisation signed JWT. opts.algorithm defaults to
  "HS256"; "HS384" and "HS512" are also supported. Claims
  pass straight through to jwt-go's MapClaims, so RFC 7519
  reserved claims (exp, nbf, iat, aud, iss, sub,
  jti) work alongside arbitrary application claims. Missing
  reserved claims aren't synthesised.
• api.jwt.view(token) decodes header + payload without
  verifying the signature and returns { header, payload, signature }. Useful for debugging auth flows or surfacing
  aud / iss to the user before deciding to trust the token.
  Both RawURLEncoding (no padding) and the padded URLEncoding
  form are accepted on input; the returned signature is the
  raw base64url segment as it appeared in the token.
• api.jwt.validate(token, secret, opts?) verifies the signature
  • standard claims and resolves with
    { valid: true, claims } or { valid: false, reason }. The
    resolve-don't-throw contract is the key design point: scripts
    branch on valid for bad signature / expired / nbf / aud
    mismatch / iss mismatch. Only structural input errors (wrong
    segment count, invalid base64, invalid JSON, empty secret /
    empty token) throw — those aren't validation failures and a
    script pattern-matching on valid: false shouldn't
    accidentally accept a garbage string.
• opts.audience / opts.issuer propagate into jwt-go's
  WithAudience / WithIssuer parser options when set; absent,
  jwt-go skips those checks.
• Unsupported algorithm names — including RS256, ES256,
  EdDSA, and the special "none" value — throw at sign /
  validate time with a named-algorithm error. Silent fallback
  to a weaker algorithm than requested would be a security
  footgun.
• TestJwt* (19 sub-tests covering: round-trip per algorithm
  HS256/HS384/HS512, view-without-secret, bad-signature
  resolve-false, expired-token resolve-false, audience and issuer
  mismatch resolve-false, four asymmetric-algo rejections, four
  malformed-input throws, empty-secret throws on both sign and
  validate).
• examples/scripts/jwt.ts walks through every form; pure stdlib
  so it runs in the CI offline subset.
• --examples step 30 covers the binding; MANUAL section 5 gains
  the api.jwt block plus a paragraph per op.
• cmd/sercon/api_docs.go grows three entries so the emitted
  api.d.ts carries hover docs.

Changed

• OUT-OF-SCOPE.md's JWT entry rewritten: HMAC support is shipped;
  the asymmetric matrix (RSA / ECDSA / EdDSA) is now an explicit
  follow-up item with notes on the JS-side key-shape design.