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/_section.md

@@ -0,0 +1,11 @@
+---
+id: log
+title: Log
+order: 6
+---
+
+Levelled, structured logging — the default way to answer "what is my game doing?" and "why did that break?", and a real step up from scattering <code>print</code> calls through your code. Each message carries a level, from <a href="log-trace"><code>trace</code></a> (most verbose) through <a href="log-debug"><code>debug</code></a>, <a href="log-info"><code>info</code></a>, and <a href="log-warn"><code>warn</code></a> to <a href="log-error"><code>error</code></a>, and a runtime threshold set with <a href="log-set_level"><code>Log.set_level</code></a> decides which ones actually appear — so a development build can be chatty and a release build quiet, without touching the call sites.
+
+Messages go to standard error, kept separate from a program's real stdout, tagged with their level. Beyond the message you can pass **structured fields** as trailing key/value pairs — <code>Log.warn("missing texture", "path", p, "id", n)</code> prints <code>[WARN] missing texture path=... id=...</code> — cheap to write and easy to grep. Numeric values (int, long) are formatted for you; the level tag is chosen at compile time, so a filtered-out level costs only a threshold comparison at runtime.
+
+Logging never touches the simulation — it writes to stderr and returns — so it has no effect on gameplay determinism or replays. This first version ships the console (stderr) sink; a rotating-file sink and an in-engine overlay sink are planned follow-ups.