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>

examples/library/logging.ludic

@@ -0,0 +1,28 @@
+# logging.ludic — Log.* levels, the threshold gate, and structured fields. Log
+# lines go to stderr, which the test harness does not capture, so this program
+# asserts the stdout-observable contract: the threshold round-trips through
+# set_level/level, and every level method (with mixed string/number fields) runs
+# without faulting. The run raises the level above `error` first so the suite's
+# output stays clean; the actual stderr sink and its gating are demonstrated in
+# the docs and verified by hand. Running it prints: 0 5 2 1
+program Logging {
+  entry {
+    print(Log.level())              # 0 — everything shown by default
+
+    Log.set_level(5)                # above error: silence the console for this run
+    print(Log.level())              # 5
+
+    # every level builds its line (including numeric fields, which are formatted
+    # for you) and is threshold-gated; none writes at level 5.
+    Log.trace("entering frame", "n", 10)
+    Log.debug("state", "hp", 100, "phase", "combat")
+    Log.info("level loaded", "name", "cavern", "entities", 42)
+    Log.warn("missing texture, using placeholder", "path", "rock.png")
+    Log.error("save failed", "code", 5)
+
+    Log.set_level(2)                # info and above
+    print(Log.level())              # 2
+
+    print(1)                        # sentinel: reached the end without crashing
+  }
+}