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/requirements.md

@@ -59,6 +59,12 @@ group enabled. (Earlier reference points: 1006 after ADR-0024
   for inspecting hints about the current input or last error.
 - [ ] **S5** Mode label and distinct border style on the input
   field communicate the current input mode at all times.
+- [ ] **S6** 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 walker diagnostics-severity model
+  (ERROR / WARNING). Advisory only — never blocks submission.
+  Designed in ADR-0027; implementation pending.
 
 ## Input field