test(benchmarks): isolated microbenchmarks for core hot-path kernels

[HanSur94]

What

Adds five focused, deterministic benchmarks that grow coverage over FastSense's performance-critical surface. Each isolates one hot-path kernel as pure computation (no figure / no rendering) and guards it with a machine-independent scaling gate — so a regression trips the gate regardless of host speed.

Benchmark	Kernel / path	Gate
bench_downsample_kernels.m	MinMax + LTTB downsampling (minmax_core_mex / lttb_core_mex)	log-log size exponent ≤ 1.3
bench_binary_search.m	range-window lookup (binary_search_mex)	per-query exponent ≤ 0.6 (catches O(log N)→O(N))
bench_violation_cull.m	fused threshold-marker detect+cull (violation_cull_mex), constant + step-function	size exponent ≤ 1.3
bench_datastore_range.m	disk range query (FastSenseDataStore.getRange)	fixed-window query time ~constant vs dataset size (catches full-scan)
bench_delimited_parse.m	CSV ingestion (delimited_parse_mex)	row exponent ≤ 1.3

Why

• LTTB downsampling and the CSV-ingestion kernel had zero benchmark coverage; the others were only exercised indirectly inside render-heavy sweeps.
• The gates are relative (scaling exponents), so they're stable across machines and CI — they catch algorithmic regressions (e.g. an indexed read degrading to a full scan, or super-linear parse creep), not absolute-time noise.

Notes

• Follows the existing bench_*.m house style (path/install() header, size sweep, warmup, median-of-runs, printed table + assert gate).
• Private wrappers are reached by cd-ing into the owning private/ folder — works in both MATLAB and Octave, unlike addpath of a private dir (which MATLAB rejects).
• All five verified to run clean and PASS in MATLAB R2025b with MEX active.
• benchmarks/.reports/coverage.md documents the ranked surface, what each run added, and the remaining gaps (e.g. EventStore persistence, LiveEventPipeline tick).
• No library / production code changed — new files only. /bench-guard will baseline these on its next run.

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