docs: add ADR-0027 — input-field validity indicator

A debounced `[ERR]` / `[WRN]` marker at the right edge of
the input row, summarising — before submit — whether the
current command would run. Backed by a small
diagnostics-severity model: the walker emits severity-
tagged diagnostics (parse outcome, schema-existence of
table / column names) that the indicator summarises and
the existing highlighting / hint layers detail. Advisory
only — submission is never blocked.

- docs/adr/0027-input-validity-indicator.md — the ADR.
- docs/adr/README.md — index entry.
- docs/requirements.md — new S6 (TUI shell).

docs/adr/README.md

@@ -32,3 +32,4 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0024 — Unified grammar tree: execution plan](0024-unified-grammar-tree-execution-plan.md) — **Accepted**, the executable spec — implemented (Phases A–F; Phase F shipped "minimal", `parser.rs` retained as the router — see the ADR's Phase F implementation note)
 - [ADR-0025 — Indexes](0025-indexes.md) — **Accepted**, `add index` / `drop index`, persistence, rebuild-table preservation, and items-list display (`C3` index portion + `S2`)
 - [ADR-0026 — Complex WHERE expressions](0026-complex-where-expressions.md) — **Accepted**, stratified recursive expression grammar (`AND`/`OR`/`NOT`, comparisons, `LIKE`, `IS NULL`, `IN`, `BETWEEN`) for `update` / `delete` / `show data` filters; `show data` gains `where` + `limit`; adds the `Subgrammar` node and a recursive `Expr` AST (`C5a`)
+- [ADR-0027 — Input-field validity indicator](0027-input-validity-indicator.md) — **Accepted**, a debounced `[ERR]` / `[WRN]` marker at the input row's right edge, backed by a walker diagnostics-severity model (parse-outcome + schema-existence); advisory, never blocks submission (`S6`)