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-error.md

@@ -0,0 +1,26 @@
+---
+id: log-error
+name: Log.error
+category: log
+kind: namespace-method
+tokens: Log.error
+sig: Log.error(msg, [key, value]...) -> void
+tip: Log at the error level (4) — a failure.
+order: 5
+ns: Log
+member: error
+---
+
+Logs <code>msg</code> at the **error** level (4), the highest — a real failure the game could not handle cleanly: a save that would not write, a required asset that could not load. It prints on stderr as <code>[ERROR] msg</code> with any structured <code>key=value</code> fields, and shows at every threshold except one set above 4. Error is the level you almost always want visible, including in release builds.
+
+Parameters:
+- `msg` — the message string
+- `key, value...` — optional trailing pairs; values may be strings or numbers
+
+```ludic
+program Error {
+  entry {
+    Log.error("save failed", "path", "slot1.sav", "code", 5)
+  }
+}
+```