Browser demo: add a Pause/Resume affordance

[DavidLiedle]

Summary

The WASM demo at https://davidcanhelp.github.io/unit/ is now alife-first:
a lone unit calls "Hello?", spawns a friend, and the colony chatters
sparsely from there. This is great for landing impressions, but a visitor
who wants to read a long bubble, take a screenshot, or just stop and look
has no way to freeze the canvas.

This issue is to add a small "pause" chip to the header that suspends the
autonomous timers (autoTick, autoSpawnCheck, loneChatterTick) and a
"resume" toggle that brings them back. Existing simulation state (units,
fitness, energy, dictionary) should be preserved across the pause — only
the autonomous drivers stop. The REPL should remain usable while paused.

Why it's a good first issue

Self-contained: lives entirely inside web/index.html and touches no
Rust, no WASM, no protocol. The autonomous timers are all setInterval
calls in one section of the file (search for setInterval(autoTick),
which makes the pause point obvious. No deep simulation knowledge needed.

It also leaves room for follow-up polish (keyboard shortcut, auto-pause
when the tab is hidden, "step one tick" while paused) that a contributor
can scope as they like.

Note: as of v0.28, autoTick also drives the signaling layer's visible
surface — COURT emissions render as signals N bubbles, the LISTEN
cue renders heard N bubbles, and mesh.drainAndRoute routes signals
into siblings' inboxes after every eval. All of these run inside
autoTick, so pausing the three existing timers stops them too — no
additional pause wiring is needed. Just be aware they exist when
verifying.

Acceptance criteria

• A pause chip appears in the #info-right header next to the
  existing chips (spawn, chatter, metrics, repl).
• Clicking it stops autoTick, autoSpawnCheck, and the
  loneChatterTick interval; the chip toggles its active style.
• Clicking again restores all three timers without losing canvas state.
• The REPL drawer continues to accept input while paused — typing
  2 3 + . still returns 5 ok.
• No new dependencies, no build step.

Where to look

• web/index.html — search for setInterval(autoTick,
  setInterval(autoSpawnCheck, and loneIntervalId = setInterval to find
  the three drivers.
• toggleMute, toggleDashboard, toggleRepl are good models for the
  chip wiring.
• The "alife-first" layout-pass section (search the file for
  LAYOUT PLAN) explains the surrounding design intent.

Nice-to-haves (not required)

• Bind the spacebar to toggle pause when the REPL drawer is closed.
• Show a small "paused" indicator overlaid on the canvas.