feat(stdlib): add Log.* — levelled, structured logging (#15)

A Log.* namespace: five levels (trace/debug/info/warn/error), a runtime
threshold, and structured key=value fields, so games get something better than
scattered print calls and release builds can go quiet without touching call
sites.

  - Log.trace/debug/info/warn/error(msg, [k, v]...)  -> stderr, "[LEVEL] msg k=v"
  - Log.set_level(n)   show only level >= n (0 = all default, 5 silences all)
  - Log.level()        read the current threshold

Fields accept strings, ints, and longs (numbers formatted automatically); the
level tag is chosen at compile time so a filtered-out level costs only a
comparison. Writes to stderr, never touching the simulation — no effect on
determinism/replays. v1 is the console sink; rotating-file and in-engine overlay
sinks are noted as follow-ups.

- examples/library/logging.ludic: asserts the set_level/level threshold
  round-trip and that every level (with mixed-type fields) runs without faulting;
  the stderr gating itself was verified by hand (warn/error emit, lower levels
  suppressed). Wired into `x test` (now 53 passed).
- docs: a new Log section + per-symbol pages; inventory and coverage pass.
- seed regenerated; `x bootstrap-cfree` fixpoint holds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/log/log-level.md

@@ -0,0 +1,24 @@
+---
+id: log-level
+name: Log.level
+category: log
+kind: namespace-method
+tokens: Log.level
+sig: Log.level() -> int
+tip: The current logging threshold.
+order: 7
+ns: Log
+member: level
+---
+
+Returns the current logging threshold — the minimum level that <a href="log-set_level"><code>Log.set_level</code></a> last set (0 by default). Use it to branch on how verbose logging is: skip building an expensive debug string when it would be dropped anyway, or show an in-game indicator that verbose logging is on. The levels are <code>trace</code> 0 through <code>error</code> 4.
+
+```ludic
+program Level {
+  entry {
+    print(Log.level())        # 0
+    Log.set_level(2)
+    print(Log.level())        # 2
+  }
+}
+```