Fix conditional visibility, panel grouping, and the settings they silently dropped

[StuartCameronCode]

Started as "the Chroma Fixes parameters aren't intuitive" and turned out to be four defects, three of them affecting most of the app.

What was wrong

visibleWhen was being ignored. It is declared only on ParameterUiConfig, so a schema that writes it one level up has it dropped at parse time and the control is always visible. That was 35 parameters across 8 schemas, plus 12 sections (UiSection has no visibleWhen either) — all written during the 2026-08-17 build-out. Beyond Chroma Fixes: Noise Reduction showed mClean and TemporalDegrain2 knobs under SMDegrain, SpotLess showed RemoveDirt's, Chroma Denoise showed CCD's under Cnr4, Deflicker showed both methods' at once.

Sections were not headings. The compact panel printed a title only for an advancedOnly section in advanced mode, so every ordinary section ran together into one flat list and a title was in practice a comment.

Three settings applied on top of corrections already made, and one switch was never read. Automatic chroma alignment measures the misalignment and corrects it, then the manual Y/C shift was applied on top. Automatic levels places black and white, then the manual input/output points graded the already-graded picture. And apply_levels — the panel's own Levels switch — never reached the worker: the block was emitted whenever a value differed from its default, so unticking it left the adjustment running with nothing on screen able to stop it.

copyWith silently reset 26 fields in six models. togglePass rebuilds a pass through copyWith, so flicking a pass switch discarded them — including the DeDot that VHS Cleanup turns on and the automatic alignment that DV Camcorder Tape turns on.

What changed

• Conditions moved to where the model reads them, section conditions pushed down onto their parameters; the panel prints section titles whenever a schema has more than one.
• Chroma Fixes regrouped into its five actual repairs (alignment, bleeding, dot crawl, rainbowing, chroma combing): a switch, the controls you would reach for, and an advancedOnly tuning section per filter. Default panel went from ~25 always-on controls to 8 switches.
• Color Correction pairs each automatic control with the manual one it supersedes, instead of collecting both in a separate "Automatic" group. Automatic levels hides the manual points it replaces but keeps gamma, which it never touches; automatic white balance leaves temperature and tint alone, because an offset still means what its label says after a correction while a mapping does not.
• Two more inert Method dropdowns removed. Color Correction (tweak/white_balance) and Crop & Resize (standard/nnedi3_2x/eedi3_2x) each declared methods that gated nothing — no parameter conditional on the choice, no model field, the converter hardcoding one value or emitting none. Crop & Resize was the sharper case: its real upscaler choice is the upscaleMethod parameter, so the dropdown was an inert duplicate of a working control.
• deblock.quant1 was ungated while DCTFilter, which does not take it, was one of the three methods offered.
• Headings that read as comments renamed after the filter whose controls they hold; three dead sections holding only method removed.
• Precedence lives in one derivation per pair (effective_apply_chroma_shift, effective_levels_points, effective_apply_levels, and their Dart twins), so the panel, the script and the pass summary cannot disagree. None clears the stored value, so switching the automatic side off restores what was set by hand.
• Pass summaries name every repair they will perform — Chroma Fixes' omitted four, Colour Correction's omitted four.

Behaviour changes worth flagging

1. A job or preset with both automatic and manual alignment set now applies only the automatic correction (previously both, i.e. a double correction).
2. A job with level values but applyLevels unticked no longer applies them. The switch is authoritative now; no built-in preset is affected.
3. Automatic levels + manual points now applies only gamma from the manual side.

Guards added

• filter_schema_curation_test: nothing may declare visibleWhen outside a parameter's ui; no section may carry one; every condition must name a parameter that exists (a condition that can never be satisfied hides a control forever — the worse failure); a schema with 2+ methods must gate at least one parameter on the choice; a parameter used by a subset of methods must name exactly those; no section may hold only method.
• chroma_fixes_panel_test, color_correction_panel_test: pump the real schemas through the real panel — the only level that shows what is actually on screen.
• parameter_copy_with_test: two passes, behavioural and source-level. The behavioural one cannot perturb a string or enum (an invented value would not decode), which is exactly the shape of the SpotLess method and Subtitles burnInPath bugs.
• preset_visibility_test: walks every built-in preset's enabled passes through the real converter and fails if a value differing from the schema default is not reachable in that preset's own state. Values hidden only by a method condition are exempt (builtin-fast selects Bwdif and sets a QTGMC preset deliberately); three advanced-only QTGMC values pass through a named allowlist, with a companion test that fails if the list goes stale.
• test_150, test_151 in the Rust suite, with Dart counterparts.

Each new guard was verified to fail when its fix is removed.

Testing

• flutter test --exclude-tags heavy — 746 pass
• cargo test — green apart from subtitle_integration_test, which needs the whisper small model that is not installed locally (unrelated to this branch)
• Ran the debug app throughout: checked the regrouped panels, the automatic/manual switching, and the headings now appearing in Deinterlace, Noise Reduction and Crop & Resize

Not in this PR

• Deinterlace's fourteen section titles — established, documented and referenced throughout CLAUDE.md; renaming them is a bigger decision than a heading tidy.
• Parameter labels mix Title Case and sentence case, sometimes within one panel, and headings make it more obvious. Several hundred strings across 21 files, so it deserves its own change.

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