docs(plan): kill the pointer that sent this session to the wrong section

`PLAN-v1.md`'s Session H block ended with "Neste: Session G", and that line is
a trap two ways over. "Session G" is a WRITTEN section at :231 — and it is
*v1.0 freeze + release*, not the axis separation. No plan section for the axis
separation ever existed; the pointer resolved to this one forward-reference
line. A session following it designs against the wrong section, which is
exactly what this session started doing before measuring the file.

The number was wrong too: 0.4.0 went to the input cap by operator choice, so
the axis separation landed under 0.5.0 instead.

STATE.md is LOCAL-ONLY and gets overwritten every session, so the plan file and
the CHANGELOG are the only durable record. Leaving the line meant the next
session would re-derive the same wrong pointer from the same text.

Also corrects a citation this session nearly propagated: the locked 0.3.1
grading table is at :288-290. `:246-252` is the okf pin discussion, and the
table at :302-309 is the *broken* v0.3.0 one — the row it asserts for an
ordinary image is `fail_secure`, the regression 0.3.1 exists to fix. Citing it
as the baseline would have inverted the check.

Adds the partial-action-map test: every level a caller leaves unnamed must fall
back rather than raise. The existing test passed a complete map, so the
fallback branch had no coverage — and a KeyError there would have been caught
by `guard` and rendered as a fail-closed with a useless reason.

716 passed; matrix 128/128 + 6/6.

Note on the one red seen while landing this: test_output.py's ReDoS wall-clock
budget failed on a loaded run (suite 37s vs 13.9s), then passed alone and in a
clean full run. It asserts `scan_output` under 2.0s, and `output.py` neither
imports `disposition` nor references it in code — its three mentions are
docstring prose — so this change cannot reach that timing. Load, as STATE
documents, not a regression.

tests/test_disposition.py

@@ -334,6 +334,22 @@ def test_custom_action_map_changes_the_action_not_the_assessment():
     assert decide(report, lenient).assessment is Risk.SEVERE
 
 
+def test_partial_action_map_falls_back_per_level():
+    # A partial override is a likely way to reach for this, so the levels left
+    # unnamed must fall back rather than raise. A KeyError here would be caught
+    # by `guard` and turned into a fail-closed — silently, with a useless reason.
+    policy = Policy(trust=Trust.UNTRUSTED,
+                    action_map={Risk.SEVERE: Disposition.QUARANTINE_REVIEW})
+
+    severe = decide(_report(_finding(severity=Severity.HIGH)), policy)
+    elevated = decide(_report(_finding(severity=Severity.MEDIUM)), policy)
+
+    assert severe.assessment is Risk.SEVERE
+    assert severe.disposition is Disposition.QUARANTINE_REVIEW   # overridden
+    assert elevated.assessment is Risk.ELEVATED
+    assert elevated.disposition is Disposition.QUARANTINE_REVIEW  # defaulted
+
+
 def test_overlays_escalate_the_assessment_not_only_the_action():
     # Compound escalation and the quarantine floor are assessment-level moves;
     # if they only moved the action, a custom action_map would silently drop