Pattern B (O(N²) rebuild-per-delta in hot paths) has no lintable
signature, unlike Pattern A's ASYNC ruff gate: `s += frag` is quadratic
in a hot loop and harmless elsewhere. The only durable prevention is
behavioral — pin the scaling SHAPE of each known hot path and fail CI
when it regresses.
New tests/perf_guards/test_pattern_b_scaling.py, three guards:
1. Streamed-text accumulation (fix in flight: #92166)
Self-normalizing ratio: time at 16k deltas over 4k deltas.
Linear ≈ 4x, quadratic ≈ 16x; main measures 9.6x → strict bound 7.0.
xfail on today's main, PASSES on the #92166 branch (verified).
2. list_sessions_rich statement count (fix in flight: #95380)
Deterministic — counts writer-connection SQL statements via sqlite
trace callback, zero timing. Bounded-constant guard xfails on main
(measured N+1: 26 stmts / 12 sessions), PASSES on #95380 (verified).
A second, weaker budget guard (≤4·N+8) passes TODAY and catches a
regression from N+1 to N·M immediately.
3. Tool-call fragment assembly (#92242 shape)
Ratio guard on the buffered-parts accumulator model; sized so the
small case takes ≥3ms (sub-ms bases jitter on CI runners).
Flake hardening: min-of-K timing, ratio thresholds with ≥2x separation
from both measured-good and measured-bad, operation counts preferred
over timing. 10/10 identical outcomes across repeated local runs.
The xfail markers are the ratchet contract: each names its fix PR and
must be removed when that PR merges, flipping the guard to enforcing.
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