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-set_level.md

@@ -0,0 +1,27 @@
+---
+id: log-set_level
+name: Log.set_level
+category: log
+kind: namespace-method
+tokens: Log.set_level
+sig: Log.set_level(n) -> void
+tip: Show only messages at level n or above (0 = all).
+order: 6
+ns: Log
+member: set_level
+---
+
+Sets the logging threshold: only messages whose level is <code>n</code> or higher are written; everything below is silently dropped. The levels are <code>trace</code> 0, <code>debug</code> 1, <code>info</code> 2, <code>warn</code> 3, <code>error</code> 4, so <code>set_level(3)</code> shows only warnings and errors, and the default of <code>0</code> shows everything. Set it once at startup — low in development, high (3, or 5 to silence all) for release — and every call site adjusts automatically. Read the current value back with <a href="log-level"><code>Log.level</code></a>.
+
+Parameters:
+- `n` — the minimum level to show (0–4; use 5 to silence every level)
+
+```ludic
+program Quiet {
+  entry {
+    Log.set_level(3)          # warnings and errors only
+    Log.info("startup")       # dropped
+    Log.warn("low memory")    # shown
+  }
+}
+```