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.

tests/profiler/behave_shell/test_parse_and_bursts.py

@@ -0,0 +1,121 @@
+"""Step 1: shard parsing + paste-burst detection.
+
+Synthetic streams covering pure-typed, pure-pasted, and mixed input,
+plus the JSONL shard line parser and its junk handling.
+"""
+from __future__ import annotations
+
+import json
+
+from decnet.profiler.behave_shell._ctx import build_session_context
+from decnet.profiler.behave_shell._parse import (
+    PasteBurst,
+    parse_shard,
+    parse_shard_line,
+)
+
+
+# ── parse_shard_line ───────────────────────────────────────────────────────
+
+def test_parse_shard_line_event() -> None:
+    line = json.dumps({"sid": "x", "t": 1.5, "ch": "i", "d": "ls"})
+    assert parse_shard_line(line) == (1.5, "i", "ls")
+
+
+def test_parse_shard_line_skips_header() -> None:
+    line = json.dumps({"sid": "x", "hdr": {"version": 2}})
+    assert parse_shard_line(line) is None
+
+
+def test_parse_shard_line_handles_blank_and_garbage() -> None:
+    assert parse_shard_line("") is None
+    assert parse_shard_line("   ") is None
+    assert parse_shard_line("not json") is None
+    assert parse_shard_line('{"t": "string-not-number", "ch": "i", "d": "x"}') is None
+    assert parse_shard_line('{"t": 1.0, "ch": "z", "d": "x"}') is None  # bad kind
+    assert parse_shard_line('"just a string"') is None
+
+
+def test_parse_shard_skips_junk_in_stream() -> None:
+    lines = [
+        json.dumps({"sid": "x", "hdr": {"version": 2}}),
+        "",
+        "garbage",
+        json.dumps({"sid": "x", "t": 0.1, "ch": "i", "d": "a"}),
+        json.dumps({"sid": "x", "t": 0.2, "ch": "o", "d": "a\r\n"}),
+    ]
+    out = list(parse_shard(lines))
+    assert out == [(0.1, "i", "a"), (0.2, "o", "a\r\n")]
+
+
+# ── paste-burst detection ──────────────────────────────────────────────────
+
+def test_pure_typed_stream_has_no_paste_bursts() -> None:
+    # Single-char input events spaced by 0.1s: pure typing.
+    events = [(i * 0.1, "i", c) for i, c in enumerate("hello world\r")]
+    ctx = build_session_context(events, sid="t-typed", source="test")
+    assert ctx.paste_bursts == ()
+    assert ctx.paste_event_count == 0
+    # IATs computed correctly
+    assert len(ctx.iats) == len(events) - 1
+    for iat in ctx.iats:
+        assert abs(iat - 0.1) < 1e-9
+
+
+def test_single_paste_event_one_burst() -> None:
+    events = [
+        (0.0, "i", "echo hello world\r"),  # 17 chars — paste class
+    ]
+    ctx = build_session_context(events, sid="t-paste-1", source="test")
+    assert len(ctx.paste_bursts) == 1
+    assert ctx.paste_event_count == 1
+    burst = ctx.paste_bursts[0]
+    assert burst.start_ts == 0.0
+    assert burst.end_ts == 0.0
+    assert burst.char_count == 17
+    assert burst.event_count == 1
+
+
+def test_two_close_paste_events_collapse_into_one_burst() -> None:
+    events = [
+        (0.0, "i", "first paste line\r"),
+        (0.05, "i", "second paste line\r"),  # within PASTE_BURST_MAX_IAT_S
+    ]
+    ctx = build_session_context(events, sid="t-paste-2", source="test")
+    assert len(ctx.paste_bursts) == 1
+    assert ctx.paste_event_count == 2
+    assert ctx.paste_bursts[0].event_count == 2
+    assert ctx.paste_bursts[0].char_count == 17 + 18
+
+
+def test_two_far_paste_events_split_into_two_bursts() -> None:
+    events = [
+        (0.0, "i", "first paste\r"),
+        (5.0, "i", "second paste\r"),  # well past the IAT cap
+    ]
+    ctx = build_session_context(events, sid="t-paste-far", source="test")
+    assert len(ctx.paste_bursts) == 2
+    assert ctx.paste_event_count == 2
+
+
+def test_typing_between_pastes_breaks_the_burst() -> None:
+    events: list = [
+        (0.0, "i", "long pasted line\r"),
+        (0.5, "i", "x"),  # single typed char interrupts
+        (0.6, "i", "another pasted line\r"),
+    ]
+    ctx = build_session_context(events, sid="t-mixed", source="test")
+    assert len(ctx.paste_bursts) == 2
+    assert ctx.paste_event_count == 2
+    # The "x" event is not a paste-class event
+    assert ctx.input_events[1][2] == "x"
+
+
+def test_paste_burst_record_is_immutable() -> None:
+    b = PasteBurst(start_ts=0.0, end_ts=1.0, char_count=10, event_count=1)
+    try:
+        b.char_count = 99  # type: ignore[misc]
+    except Exception:
+        pass
+    else:
+        raise AssertionError("PasteBurst should be frozen")