docs(session): scope the zero-band geometry claims to the default compact threshold

The drain comment and the architecture docs stated the zero-budget band
relative to the auto-compact threshold as if 0.8 were universal
("well below the auto-compact threshold"); with an operator-set
auto_compact_pct under the ~70% zero point the claim reads inverted.
State the geometry against the DEFAULT threshold and make explicit what
was always true of the mechanism: the trigger's predicate is the
exhausted budget itself, never a threshold, so with low thresholds the
owed path compacts first and the trigger is its bail/insufficient
backstop.

docs/architecture.md

@@ -985,10 +985,14 @@ context budget, and three guarantees apply when that budget reaches zero
   successful-looking trim).
 
 A zero budget also triggers one mid-turn auto-compaction before results are
-sized: the response reserve zeroes the budget well below the auto-compact
-threshold (near 70% fullness with `max_tokens ≥ context_window/4`), and
-without this trigger a session could idle in that band indefinitely with
-every tool result floored or dropped.
+sized. With `max_tokens ≥ context_window/4` the response reserve zeroes the
+budget near 70% fullness — below the default 80% auto-compact threshold —
+and without this trigger a session could idle in that band indefinitely
+with every tool result floored or dropped. The trigger keys on the
+exhausted budget itself, not on any threshold, so it composes with any
+operator-set `auto_compact_pct`: with thresholds below the zero point the
+ordinary owed-compaction paths fire first and this trigger degrades to a
+backstop for the cases where they bailed or freed too little.
 
 ---
 

turnstone/core/session.py

@@ -6431,14 +6431,20 @@ class ChatSession:
                     and zero_budget_compact_attempts < _ZERO_BUDGET_COMPACT_CAP_PER_SEND
                     and self._generation == my_generation
                 ):
-                    # Zero tool-result budget wedges the loop BELOW the owed
-                    # thresholds: with max_tokens ≥ context_window/4 the
-                    # response reserve zeroes the budget near 70% fullness,
-                    # well under auto_compact_pct, and a stalled model
-                    # appends too little to ever cross it — so without this
+                    # Zero tool-result budget can wedge the loop BELOW the
+                    # owed thresholds: with max_tokens ≥ context_window/4
+                    # the response reserve zeroes the budget near 70%
+                    # fullness — under the DEFAULT auto_compact_pct, and
+                    # further under any raised one — while a stalled model
+                    # appends too little to ever cross it, so without this
                     # trigger the session can sit in the zero band
                     # indefinitely while every tool result is floored or
-                    # dropped (#883).  Zero budget is itself
+                    # dropped (#883).  The predicate is the exhausted
+                    # budget itself, never a threshold, so it composes
+                    # with any operator-set auto_compact_pct: thresholds
+                    # below the zero point compact via the owed path
+                    # first, and this branch is its bail/insufficient
+                    # backstop.  Zero budget is itself
                     # compaction-owed evidence.  ``auto=True`` WITHOUT
                     # ``threshold_pct``, exactly like the ctx-overflow
                     # retry: no threshold was evaluated, so the notice must