feat(profiler/behave_shell): asciinema parser + paste-burst detection

BEHAVE-EXTRACTOR.md Phase A Step 1. Lays the shared primitives that
Steps 2-3 (motor.input_modality, motor.paste_burst_rate) will
consume:

* parse_shard_line / parse_shard turn a shard JSONL line/file into
  AsciinemaEvents, skipping headers and malformed records.
* PasteBurst dataclass + _detect_paste_bursts group consecutive
  paste-class input events (len(d) >= 4 chars per the prototype's
  empirical floor) into contiguous bursts, splitting on IAT gaps
  larger than PASTE_BURST_MAX_IAT_S (200ms).
* SessionContext now carries iats and paste_bursts derivations.
* Threshold constants harvested from
  BEHAVE/prototype_extractors/shell/extract.py — calibrated against
  the five 2026-05-02 shards.

Tests cover pure-typed, pure-pasted, mixed streams; close vs far
paste events; typed events breaking a burst; PasteBurst immutability;
and the JSON parser's junk handling.

decnet/profiler/behave_shell/_thresholds.py

@@ -1,11 +1,37 @@
 """Numeric thresholds for BEHAVE-SHELL primitive classification.
 
-Each constant added here cites its calibration source. When the
-registry's ``notes:`` field disagrees with a constant in this file the
-registry is authoritative — fix the constant and re-run the
-calibration grid.
+Each constant cites its calibration source. When the registry's
+``notes:`` field disagrees with a constant here, the registry is
+authoritative — fix the constant, re-run the calibration grid.
 
-Step 0 ships this file empty by design; thresholds land alongside the
-feature functions that consume them (Steps 1+).
+Empirical thresholds inherited from the BEHAVE prototype extractor
+(``BEHAVE/prototype_extractors/shell/extract.py``); see lines 40-90 of
+that file for the calibration history. Any change here must keep the
+five-class grid green.
 """
 from __future__ import annotations
+
+# ── paste-burst detection (Step 1) ──────────────────────────────────────────
+# A single input event with ≥ PASTE_MIN_CHARS_PER_EVENT chars is the
+# paste-class proxy used by the prototype; xterm-kitty / iTerm / VS Code
+# pastes arrive as one bulk write.
+PASTE_MIN_CHARS_PER_EVENT: int = 4
+
+# Consecutive paste-class events arriving within this IAT collapse into
+# one PasteBurst record. 200ms is the prototype's IKI burst cap.
+PASTE_BURST_MAX_IAT_S: float = 0.20
+
+# ── motor.input_modality (Step 2) ───────────────────────────────────────────
+# Paste-event ratio thresholds. ≥ 40% paste events → "pasted" (LLM-driven);
+# ≤ 5% → "typed" (human at the keyboard); in between → "mixed".
+# Lowered from 0.5 after the 47.6% case in sessions-2026-05-02-with-llm.jsonl
+# was clearly LLM-driven but missed the 0.5 floor.
+MODALITY_PASTED_MIN: float = 0.40
+MODALITY_TYPED_MAX: float = 0.05
+
+# ── motor.paste_burst_rate (Step 3) ─────────────────────────────────────────
+# Same paste-event ratio re-bucketed for the "how often does the operator
+# paste" axis. Coarser than input_modality on purpose: this primitive is the
+# habit signal, input_modality is the dominant-channel signal.
+PASTE_RATE_HABITUAL_MIN: float = 0.50
+PASTE_RATE_OCCASIONAL_MIN: float = 0.10