Convert the schema slice to signals (schema/schemaError/schemaFilter) — Phase 1 of #88

[BorisTyshkevich]

Part of #68 (Phase 1) — the final slice of the #88 reactivity migration. Links to roadmap #68; foundation in docs/ADR-0001-reactivity.md + CLAUDE.md rule 5.

Goal

Finish the #88 migration by converting the schema panel's state — schema, schemaError, schemaFilter — to @preact/signals-core signals, so the tree repaints via an effect() like every other slice. The schema panel stays imperative (no Preact — the spike was rejected, ADR-0001 addendum); the in-place mutation that made this the awkward slice is replaced with reference-replacing updates, not a component model.

Files / scope

• src/state.js — schema / schemaError / schemaFilter → signal(...).
• src/ui/app.js — loadSchema / loadColumns write via .value; an effect() in createApp reads the schema signals and calls renderSchema(app) (replacing the scattered manual renderSchema calls); rebuildCompletions / updateBanner read .value; the schema-filter input sets schemaFilter.value.
• src/ui/schema.js — renderSchema reads .value; expand toggles + lazy column load update by reference (see below), not in place.
• Tests — sweep tests/unit/{schema,app,state}.test.js to .value (the unit pattern from the prior slices).
• Non-goals: not a Preact rewrite (rejected — ADR-0001 addendum); no schema-tree feature changes; the e2e editor harness is unaffected (no dep added).

Key implementation

• Mirror the established slice pattern (tabs / sidePanel / resultView / libraryName): field → signal, reads/writes via .value, one effect() in createApp replaces manual renderX invalidation, batch() for multi-signal updates.
• Kill the in-place mutation (the reason this slice was awkward): replace the expandedTables Set with a Set-valued signal updated immutably (expanded.value = new Set(expanded.value).add(key)); replace tb.columns in-place writes either with a Map-valued signal (cols.value = new Map(cols.value).set(key, …)) or by keeping tb.columns as a completion cache and bumping the schema signal. Pick the lower-churn option and document it in the PR.
• Keep renderSchema imperative (rebuild the tree on the effect) — the documented exception to "no components" (ADR-0001 / CLAUDE.md rule 5).

Acceptance criteria

• schema / schemaError / schemaFilter are signal(...); all reads/writes go through .value.
• No in-place mutation drives rendering — expand state + loaded columns update by reference; an effect() repaints the tree (no leftover manual renderSchema calls beyond any deliberate data-path one).
• Schema load, load-error banner, lazy column load, filter, expand/collapse, double-click insert, shift-click SHOW CREATE, and drag all behave exactly as before — no user-facing change.
• npm test green at the per-file 100% gate; npm run build clean; e2e green on all three engines.
• CHANGELOG [Unreleased] + Roadmap to 1.0.0 #68 Phase 1 reconciled; with this slice done, Adopt reactivity via @preact/signals-core, incrementally (ADR-0001) #88 can close.

Re-evaluation trigger

If reference-replacing the tree proves as forgettable as the old manual renderSchema calls (the "relocated pain" the ADR warned about), reconsider whether the schema panel stays a documented imperative exception or warrants the (currently-rejected) component model — but only via a fresh ADR.

Tracking

Phase 1 of #68; the last slice of #88 (closes the migration). The rejected Preact alternative is preserved on spike/preact-schema as evidence.