feat(profiler/behave_shell): emit motor.error_correction

BEHAVE-EXTRACTOR.md Phase B Step B.3. Replaces the prototype's
two-line "0 vs >0 backspaces" placeholder with a backspace-timing
classifier that honours the registry's full vocabulary.

* SessionContext gains backspace_count, backspace_iats (IAT from
  each backspace back to the preceding non-backspace input event),
  and kill_line_count (^U / ^W). Built by _scan_correction_signals,
  which retains only counts and timing aggregates — no character
  data leaves the helper, in line with the BEHAVE PII discipline.
* _features/motor.py:error_correction(ctx) emits one Observation
  in {immediate, deferred, absent, route_around}.
  - 0 backspaces + ≥1 ^U/^W → route_around (rewrite, not correct)
  - 0 backspaces + 0 kill-lines → absent
  - backspaces with median IAT ≤ 500 ms → immediate
  - slower → deferred
  Confidence 0.65 / 0.65 / 0.55 / 0.55.
* < 3 inputs → skip emit.
* Calibration grid widened to include motor.error_correction;
  green across all five shards.

Tests cover all four buckets, the < 3 inputs skip, and the PII
regression (raw command body never appears in the serialised
observation).

decnet/profiler/behave_shell/_ctx.py

@@ -51,6 +51,11 @@ class SessionContext:
     # Step B.1 derivations — typing bursts (IATs split at think-pauses)
     typing_bursts: tuple[tuple[float, ...], ...] = field(default_factory=tuple)
 
+    # Step B.3 derivations — error-correction signals
+    backspace_count: int = 0
+    backspace_iats: tuple[float, ...] = field(default_factory=tuple)
+    kill_line_count: int = 0
+
 
 def _detect_paste_bursts(
     inputs: list[AsciinemaEvent],
@@ -106,6 +111,37 @@ def _detect_paste_bursts(
     return tuple(bursts), paste_count
 
 
+_BACKSPACE_CHARS = ("\x7f", "\x08")
+_KILL_LINE_CHARS = ("\x15", "\x17")
+
+
+def _scan_correction_signals(
+    inputs: list[AsciinemaEvent],
+) -> tuple[int, tuple[float, ...], int]:
+    """Walk input events char-by-char, count backspaces / kill-lines /
+    timing IATs.
+
+    PII discipline: only counts and IATs leave this function — no
+    character data is retained or returned.
+    """
+    backspace_count = 0
+    kill_line_count = 0
+    iats: list[float] = []
+    last_non_bs_t: float | None = None
+    for t, _kind, data in inputs:
+        for c in data:
+            if c in _BACKSPACE_CHARS:
+                backspace_count += 1
+                if last_non_bs_t is not None:
+                    iats.append(max(0.0, t - last_non_bs_t))
+            elif c in _KILL_LINE_CHARS:
+                kill_line_count += 1
+                last_non_bs_t = t
+            else:
+                last_non_bs_t = t
+    return backspace_count, tuple(iats), kill_line_count
+
+
 def _split_typing_bursts(iats: tuple[float, ...]) -> tuple[tuple[float, ...], ...]:
     """Split a flat IAT sequence at gaps > IKI_THINK_MAX_S.
 
@@ -200,6 +236,7 @@ def build_session_context(
     )
     paste_bursts, paste_count = _detect_paste_bursts(inputs)
     typing_bursts = _split_typing_bursts(iats)
+    backspace_count, backspace_iats, kill_line_count = _scan_correction_signals(inputs)
     commands = _segment_commands(inputs)
     inter_cmd_iats = tuple(
         max(0.0, commands[i + 1].start_ts - commands[i].end_ts)
@@ -226,4 +263,7 @@ def build_session_context(
         inter_cmd_iats=inter_cmd_iats,
         output_per_cmd=output_per_cmd,
         typing_bursts=typing_bursts,
+        backspace_count=backspace_count,
+        backspace_iats=backspace_iats,
+        kill_line_count=kill_line_count,
     )