perf: incremental era rebuilds — skip unchanged months

rebuild_eras() re-digested EVERY month from scratch on every coherence pass,
including old months whose sessions never change — ~17 redundant 32B calls per pass
(a big slice of the ~40-min consolidation grind + MI50 heat). Now it compares each
month's current session count to the stored era and only rebuilds changed months
(force=True still does all). Report gains built/skipped counts.

test_era.py: builds all first pass, skips unchanged, rebuilds only a month that
gained a session, force rebuilds all. Suite 99 green, ruff clean.

(Profile rebuild re-reading all 851 sessions every pass is the bigger remaining
hog — separate, harder fix.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_era.py

@@ -0,0 +1,44 @@
+"""Era rollups: only re-digest months whose session count changed (incremental)."""
+from __future__ import annotations
+
+import importlib
+
+import pytest
+
+from lyra.memory import Era
+
+
+@pytest.fixture
+def era(monkeypatch):
+    import lyra.era as era
+    importlib.reload(era)
+    return era
+
+
+def test_rebuild_eras_is_incremental(era, monkeypatch):
+    by_month = {"2025-01": ["a", "b"], "2025-02": ["c"]}
+    stored: dict[str, int] = {}
+    built: list[str] = []
+
+    monkeypatch.setattr(era.memory, "summaries_by_month", lambda: dict(by_month))
+    monkeypatch.setattr(era.memory, "list_eras",
+                        lambda: [Era(m, "x", c, "t") for m, c in stored.items()])
+    monkeypatch.setattr(era.memory, "store_era",
+                        lambda month, content, n: (stored.__setitem__(month, n), built.append(month)))
+    monkeypatch.setattr(era, "_digest_month", lambda gists, backend: "digest")  # no LLM
+
+    r1 = era.rebuild_eras(backend="local")               # first pass: both built
+    assert r1["built"] == 2 and r1["skipped"] == 0
+
+    built.clear()
+    r2 = era.rebuild_eras(backend="local")               # nothing changed: all skipped
+    assert r2["built"] == 0 and r2["skipped"] == 2 and built == []
+
+    built.clear()
+    by_month["2025-02"].append("d")                       # one month gains a session
+    r3 = era.rebuild_eras(backend="local")
+    assert r3["built"] == 1 and r3["skipped"] == 1 and built == ["2025-02"]
+
+    built.clear()
+    r4 = era.rebuild_eras(backend="local", force=True)   # force rebuilds all
+    assert r4["built"] == 2