fix(scripting): apply step params in place instead of rebuilding from display text

[fuzzzerd]

What

FmScript.Apply routed param maps through display-text synthesis (SynthesizeHrParams) and rebuilt the step with StepDisplayFactory.TryCreate, replacing the instance. That made every update destructive: params not named in the map reset to constructor defaults, state the display line can't carry (dialog geometry, button configuration, passthrough XML) was wiped, and HrOnly / variant / value-type slots were dropped with no error. A Value-only update on Set Variable [ $x ; Value: 1 ] produced Set Variable [ Value: 2 ; Value: ] — the synthesized token mis-bound positionally into the Name slot.

How

• Add ScriptStep.ApplyParam(name, value): sets one shape-bound public property in place and returns an error message or null. The ScriptStep<TSelf> default dispatches to the new StepParamApplier, the validating counterpart of StepDisplayParser — unknown names, non-On/Off booleans, and out-of-range enum values return errors instead of silently dropping.
• Slots the typed conversion cannot set — HrOnly, variant / value-type / list slots, emit-only wire projections (e.g. Perform Script's ParameterBeforeScript), and enums whose DisplayValues have no paired ValidValues — route through the step's own display-token grammar generically, with no per-step wiring: the synthesized token is validated against a blank probe (it must produce a wire-level change, so unrecognized values fail loudly), then re-parsed onto the live instance together with its current display tokens. Same-label current tokens are dropped and both token positions are tried, so first-match and last-match parser styles both take the new value; a token already present is an idempotent success.
• Structured slots additionally accept their wire element as an XML fragment value (<SortList>…</SortList>, <Query>…</Query>): the fragment replaces the step element's same-named children and the merged element is re-read in place through PopulateFromXml — so typed readers normalize it to canonical wire form. The fragment root must match the slot's wire element and the re-emitted step must retain it, otherwise the original state is restored and an error returned.
• ApplyUpdate mutates Steps[i] in place; properties the map does not name keep their values. ApplyAdd constructs a blank POCO through the registry factory, applies params the same way, and no longer inserts when a param errors. SynthesizeHrParams is deleted.
• ShowCustomDialogStep's InputFields slot gains HrLabel = "Inputs" so label-addressed lookups resolve to the token form its parser recognizes.

Catalog audit

A probe swept all 438 (step, display slot) pairs in the catalog, attempting to set each param through Apply with kind-appropriate values (On/Off for booleans, declared enum values, calc text, and a wire-element XML fragment for structured slots), verifying against the emitted XML. On the old path, 68 of the 438 were accepted and silently dropped or mangled; none returned an error.

With this PR:

outcome	pairs
apply correctly	423
rejected with an actionable error	14
quiet edge case	1

The 14 rejected pairs and the 1 quiet case

Real gap — wire lives nested inside a wrapper element the fragment graft cannot address:

• Configure Prompt Template — TemplateName, SQLPrompt, NaturalLanguagePrompt, FindRequestPrompt, RAGPrompt

Internal wire-alias names — duplicates of params that work under their friendly name, or state with no addressable display form:

• Dial Phone — UseDialPreferencesValue
• Go to Layout / Go to Related Record — LayoutDestination (the target is settable via Layout)
• Go to Record/Request/Page — ExitWire, LocationCalcWire
• Perform Script on Server with Callback — ScriptWire, CalculatedWire

Audit artifacts — the params behave correctly; the probe's synthetic values can't show it:

• Create PDF — CreatePDFFile accepts real fragments; the probe's placeholder fragment normalizes to the step's defaults, so the diff registers as unchanged
• Send DDE Execute — ContentType correctly rejects a fragment aimed at a typed enum whose only valid value is the default

Quiet case: Go to Portal Row's CalculationWire write lands on the property but only emits once RowPageLocation is ByCalculation.

Test plan

• 29 FmScriptApplyTests cover in-place instance identity, preservation of unmentioned params and XML-only state, loud errors (unknown param, invalid boolean, null value, unaddressable slot, wrong fragment root, malformed fragment), Perform Script targets and parameter, Show Custom Dialog buttons/inputs, Go to Layout targets, first-match parser ordering, idempotent re-application, and XML-fragment set/replace for SortList and Query
• dotnet test SharpFM.sln — 2343 passed, including the display round-trip contract suites

[github-actions]

Test Results

✔️ Tests 2343 / 2343 - passed in 15.6s
✔️ Coverage 80.31% - passed with 70% threshold
📏 11693 / 13732 lines covered 🌿 3676 / 5406 branches covered
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✏️ updated for commit b5f8150