shakedown: wave 12: auto-select preset from detected hardware (--target now optional)

[ugglr]

You shouldn't have to type your SKU when the machine already knows it. ./run with no --target now auto-selects the matching preset from the detected hardware, so the right chassis class, inventory asserts, and golden baseline all apply with nothing to type.

What this adds

• Preset auto-selection. With no --target, the orchestrator matches the machine's chip / memory / model against the presets in targets/ and uses the one that fits (printing which). Exactly one match wins; zero or several falls back to the prior chassis-auto-detect-with-asserts-skipped behavior.
• --target becomes an optional override for asserting an expected SKU ("I ordered a Max, not a Pro") or forcing a specific preset.
• The one-command examples across README, the Store Day Checklist, and the Benchmark Reference drop the manual --target.

Why it's safe

The match uses the SAME sources as the inventory asserts: chip_type/cpu_type and round(hw.memsize / 1024^3) and machine_model + machine_name. So an auto-selected preset always passes its own asserts (no "matched then failed" inconsistency). The guard [[ -z TARGET && -z CHASSIS_CLASS ]] preserves every existing explicit-input path unchanged: pass --target or set CHASSIS_CLASS and auto-select stands down.

Why python, not bash

The match parses JSON presets, reads sysctl hw.memsize as an int, and globs a directory. macOS ships no jq and bash can't do this cleanly, so it goes through Python exactly like every other JSON touch in the repo.

Verification

Live on this Intel box: ./run auto-selected mbp-16-intel-2019 (chassis intel-laptop) from the detected i7 / 32 GB / "16" model; an explicit --target mbp-16-m5-max-64 overrode it with no auto-match. Unit-tested the matcher: single match wins, ambiguous (2 presets) falls back, wrong-memory and unknown-chip fall back. Lint clean (shellcheck, all PYEOF heredocs ast.parse, zero em dashes). Passed dad review: simplicity ship-it, correctness zero-defects (the auto-selected-always-passes-asserts claim verified source-by-source), consistency one-voice.