0.6.0.0 batch: the small-fix backlog

[aoinoikaz]

Closes #18, #19, #21, #16, #22; part of #20 (the remaining item, entry names as bytes, stays open for 0.7.0.0 - it is a breaking rework that interacts with the fresh checkName guards). Rides the 0.6.0.0 major already opened by #26, so the batch ships in one release.

#18 - store-path names reject dot segments. parseBaseName accepted ".", "..", ".-x", "..-y", so paths no Nix client parses could be stored and signed. The rule is upstream's checkName dot rule - the first dash-separated component may not be "." or ".." - with ".config-1.0"-style names staying valid. Same rule nova-nix enforces at its parse and construction boundaries.

#19 - size fields are length-bounded before parsing. NarSize/FileSize parsed digit-by-digit into an unbounded Integer, quadratic in field length, before any consumer looked at the value. Sizes are uint64 on the wire (at most 20 digits, named constant); longer fields reject first, making the parse cost constant. The uint64 maximum itself still parses, pinned.

#20 (two of three) - executable marker must be empty; duplicate scalar keys last-wins. The parser accepted any marker value where the format fixes the empty string, and duplicated narinfo keys resolved first-wins where upstream's assign-as-read parser takes the last. lookupFirst is now lookupLast (Sig stays the intentionally repeatable key via lookupAll).

#21 - SecretKey no longer derives Show or Eq. The derived Show rendered the raw Ed25519 bytes through any enclosing Show - a config record, a debug trace - and the derived Eq compared secret material in input-dependent time. Show now renders the key name plus a redaction marker (deliberately not read-back-able); Eq compares the bytes with Data.ByteArray.constEq. The name compares normally - it is public. This also covers the nova-nix PushConfig exposure noted in its class G register.

#16 - the fingerprint sort test pins the whole references field. The old isSuffixOf needle also matched the pre-fix unsorted rendering; the needle now includes the leading field separator, so a regression to unsorted output fails.

#22 - the landing page's store scan is TTL-bounded. New NovaCache.Server.newTTLCache (monotonic clock, so wall-time jumps cannot starve the refresh; concurrent refreshes may double-run the idempotent read). The bundled server counts the store at most once per minute; the unauthenticated root route does bounded work per request regardless of store size. landingResponse takes the count action injected, keeping the library brand-free.

cabal test passes (all 9 groups), -Werror build with the server flag clean, ormolu and hlint clean, cabal check and sdist clean.