feat: ADR-0035 Amendment 1 — drop composite UNIQUE; friendlier drop-column + generic-error wording

F1/F2/F3 from the whole-Phase-4 /runda (handoff-42 §3):

- F3: drop an anonymous composite UNIQUE via a derived, engine-neutral
  name `unique_<cols>` — recomputed live, nothing persisted, reusing the
  existing `DROP CONSTRAINT <name>` grammar (no new syntax/metadata, the
  §4g anonymity decision intact). A name matching more than one UNIQUE is
  refused as ambiguous, never guessed. One undo step. `describe`
  annotates each composite UNIQUE with its name.
- F1: dropping a column a composite UNIQUE covers is refused up-front
  with the derived name + the actionable drop command (was an unhelpful
  generic engine refusal).
- F2: contextless friendly_message() no longer leaks a literal `{table}`
  in the generic hint (new `error.generic.hint_no_table`, selected when
  no table is in context). The table-ful path is unchanged.

Docs: ADR-0035 Amendment 1 + Status + README index + plan
docs/plans/20260526-adr-0035-composite-unique-drop-f1f2f3.md.
Tests: +5 (drop-by-name, ambiguous-refused, one-undo-step, F1 guard,
F2 no-leak) + a describe-render assertion. 1922 pass / 0 fail / 0 skip;
clippy clean.

src/friendly/keys.rs

@@ -109,6 +109,7 @@ pub const KEYS_AND_PLACEHOLDERS: &[(&str, &[&str])] = &[
     // ---- Generic engine refusal ----
     ("error.generic.headline", &["operation"]),
     ("error.generic.hint", &["table"]),
+    ("error.generic.hint_no_table", &[]),
     // ---- Invalid-value errors (pre-engine, single-line) ----
     (
         "error.invalid_value.arity.headline",