feat: add CLI falsifiers and exit tables to six examples

[TMHSDigital]

Summary

Pilot retrofit of six examples that already had measured assertions but no CLI falsifier. Each flag feeds degenerate input into an existing check. Default smoke path is unchanged. README exit tables document every nonzero code the script already returned; no codes were renumbered.

--same-axis / --flat-source shape. Not skip-flags.

Example	Flag	What it changes	Lands at
car-mirror-symmetry	--no-mirror	Mirror X off	5
lod-decimate-chain	--no-decimate	no Decimate on LOD copies	6
custom-normals-shade	--mismatch-angle	mark sharp at 20°, audit 30°	5
gltf-export-roundtrip	--no-yup	export_yup=False	9
image-pixels-testcard	--wrong-origin	write the card top-down	4
gn-modifier-inputs	--same-scale	write 1.0 to every modifier	7

--api on gn-modifier-inputs is unchanged. It still selects a valid path (auto) or the wrong series (dict/rna). --same-scale is the portable break.

Live-run table

Binaries: .scratch/blender-4.5.11-windows-x64 (4.5.11 LTS, 4db51e9d1e1e), .scratch/blender-5.1.2-windows-x64 (5.1.2, ec6e62d40fa9), .scratch/blender-5.2.1-windows-x64 (5.2.1 LTS, 9e2066aef7ef). Check-only, no --output. 5.1 evidence in this table is local, not CI. CI 5.1 is the needs-5.1 smoke job.

Example	Path	4.5.11	5.1.2	5.2.1
car-mirror-symmetry	default	0	0	0
car-mirror-symmetry	--no-mirror	5	5	5
lod-decimate-chain	default	0	0	0
lod-decimate-chain	--no-decimate	6	6	6
custom-normals-shade	default	0	0	0
custom-normals-shade	--mismatch-angle	5	5	5
gltf-export-roundtrip	default	0	0	0
gltf-export-roundtrip	--no-yup	9	9	9
image-pixels-testcard	default	0	0	0
image-pixels-testcard	--wrong-origin	4	4	4
gn-modifier-inputs	default	0	0	0
gn-modifier-inputs	--same-scale	7	7	7

Observed errors (same on all three versions):

• --no-mirror: evaluated body 126 verts ≠ 2n−c 224
• --no-decimate: evaluated tris 2260 ≥ base 2260
• --mismatch-angle: Shell sharp set 4 extra / 0 missing of 48 expected
• --no-yup: POSITION bounds deviate 8.700e-01 from axis-converted bbox
• --wrong-origin: byte round-trip error 0.9215687 > 0.0019618
• --same-scale: readback 1.0 ≠ intended 2.0 on Scale2

None of the six resisted falsification. No assertion was loosened. Exit 9 on gltf is the bbox check; legal.

Claim labels

• Live-run-proven: default and falsifier exit codes on 4.5.11, 5.1.2, 5.2.1 (table above).
• Inspection-only: README tables listing codes the render path can return (--output not exercised here). Smoke catalog rows unchanged (falsifiers must not run on the default smoke path).
• 5.1: local binaries above. CI 5.1 = this PR's labeled smoke job.

Explicitly not in this PR

• The other 35 no-falsifier examples.
• swatch-grid --no-verify (skip-flag defect; next batch).
• Pathology/sidecar five, bake-normal-high-to-low, catalog.json, gallery assets.
• license: ND terms conflict with documented snippet usage #131, ci: report-mode check that every nonzero exit is documented in its own script README #138, VERSION / CHANGELOG.md.

Test plan

• Validate + drift-check green
• Blender smoke 4.5 and 5.2 green on the default path
• needs-5.1 5.1 smoke green
• Socket checks green
• Do not merge from this phase