fix(check-design-reaches-app): resolve the model dir via find_model_dir; unreadable knob file is a FAULT, never clean

[MendixMau]

What changed and why (one paragraph)

The knob pass read theme/web/custom-variables.scss relative to the working directory. When it found nothing it printed "knobs bound 0 of 0" under clean, exit 0. #162 added a one-level app/*.mpr probe to the same lines, which fixes the common two-tree case. This PR replaces that probe with find_model_dir from _common.sh ($PROJECT_ROOT, $MPR_FILE, the app/ probe), and adds an app/theme / app/deployment probe for a tree with no .mpr. It also covers what #162 did not: a missing knob file, or one that declares no knobs, is now FAULT … UNMEASURED and exit 2. The knob file can also be passed as a third argument. This replays branch fix/design-reaches-app-two-tree (b773795) on current master, keeping #162's pass 2b (tokens valued) intact. The old branch is left untouched.

Field evidence

Original run (from the source branch): a Stage 5 obligation pass on a two-tree Mendix 11.14 app. With no arguments, knobs went from 0 of 0 before to 26 of 30 after, and tokens were 157 of 157.

Re-run on current master, against a scratch Mendix 11.13 app with a real mxbuild-compiled theme.compiled.css (never a real project model):

Layout	master cee5c9a	this PR
single-tree	knobs 0 of 46, css-vars true, exit 1	same, and prints the knob file used
two-tree, .mpr under app/	knobs 0 of 46, css-vars true, exit 1	same
two-tree, no .mpr	exit 2 "no BUILT stylesheet found"	finds app/deployment, knobs 0 of 46, exit 1
knob file missing, tokens all arrived	"clean, 2 warning(s)", exit 0, knobs 0 of 0	FAULT UNMEASURED, exit 2

That last row is the defect: a pass that read nothing, reported as clean. When violations exist too, exit 1 wins over the fault, as before. Both results are non-green.

Checklist

• No client data: the source-project name was removed from the CHANGELOG line and the commit message; diff grepped, leak guard passes
• Size cap: 4 files, +167/−28
• Test tier: T2. Field run above on both layouts; fixture cases 9–10 in tests/wave2/test-design-reaches-app.sh come from the source branch and run in CI
• Instrument rules: both layouts probed (rule 2); golden capture in fixtures/design-reaches-app/CAPTURE.md (rule 1); field run cited (rule 4)
• Routing row: n/a
• CHANGELOG line: yes, under Unreleased
• Bug entries: n/a

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