M13: seed display + copy + roll in the HUD

[TheAmericanMaker]

Wave 1 final item: shareable, replayable runs.

What

• HUD row gains a seed number input (0..2^32-1) that always shows the
  running world's seed, plus Copy seed and New seed buttons.
• Type a seed + Enter (or blur) → the world restarts under that seed
  (all-time records carry over, fx/shake/tornado/inspect clear).
  An empty entry reverts to the running seed without resetting.
• New seed rolls a uniform 32-bit seed (crypto.getRandomValues) and
  restarts.
• Reset world now restarts the current seed (was hard-coded 1).
• Copy uses navigator.clipboard with an execCommand fallback and flashes
  "Copied!" / "Copy failed".
• Splash legend gains a Seed line; number input gets dark styling.

Sim randomness is untouched — everything still flows through world.rng;
the roll lives in the UI layer only, so the M12 determinism guard is
unaffected (74/74 green locally).

Verified

• node --test "tests/*.test.js" → 74 pass / 0 fail
• /tmp/ui-smoke.mjs → 22/22 (new M13 block: display, typed seeds 7 /
  max uint32 / 0, empty-entry revert, reset-keeps-seed, roll, copy feedback)
• Headless Chromium (Playwright): seed row visible at boot (paused), typed
  seed 424242 restarts at tick 0 and runs under it after Resume, New seed
  rolls a different uint32, Reset keeps it, copy feedback + clipboard
  write, empty entry reverts — zero console errors.

Created by the Qwen3.8-27B (local) agent running in opencode, on behalf of TheAmericanMaker.