docs: ADR-0032 Amendment 2 + §10.6 regression tests

Amendment 2 records the §10.6 fixup-pass mechanism choice. §10.6
prescribes "rewriting the highlight class" on projection-list
idents at end-of-walk; the actual implementation uses a different
mechanism that achieves the identical user-visible behavior:

1. 2d's two-pass schema-existence diagnostic collects every FROM
   binding from the matched path first, then resolves projection
   idents against the complete scope. The post-walk re-resolve
   §10.6 calls for, just embedded in the diagnostic emitter.

2. input_render.rs's diagnostic-overlay path colors each
   diagnostic span Error/Warning, achieving the visual change
   §10.6 describes without needing a new HighlightClass variant.

The completion-mid-typing piece is improved by the §10.5
look-ahead probe (sub-phase 2e earlier).

Four new regression tests in `projection_before_from_tests` pin
the behavior so a future refactor can't silently regress it:
correct ident resolves silently, unknown ident flags via
diagnostic on its span, multi-projection only flags unknowns,
projection-without-FROM is silent.

ADR index entry updated to reference Amendment 2.

Test totals: 1424 → 1428 passing (+4). Clippy clean.

docs/adr/0032-sql-select-grammar.md

@@ -1390,6 +1390,96 @@ This amendment narrows the honest limitation in §12 from
 and recursive CTE result columns" — a tighter, factually
 verified carve-out.
 
+## Amendment 2 — §10.6 fixup-pass mechanism (2026-05-20)
+
+§10.6's prescription for the post-walk fixup is written in
+terms of "rewriting the highlight class" on projection-list
+`Ident` terminals — downgrading "column" → "unknown identifier"
+when an ident doesn't belong to the eventual `from_scope`, or
+upgrading the reverse direction once a `FROM` is typed. The
+implementation chose a different mechanism that achieves the
+identical user-visible effect; this Amendment records the
+choice so a reader of §10.6 doesn't go looking for a literal
+`per_byte_class` rewrite step that does not exist.
+
+### Mechanism actually used
+
+Two pieces, both already in the codebase by the end of
+sub-phase 2d:
+
+1. **Two-pass schema-existence diagnostic.** The 2d rewrite of
+   `schema_existence_diagnostics` (`src/dsl/walker/mod.rs`)
+   runs a pre-pass over the matched path that collects every
+   `IdentSource::Tables` / `cte_name` / `table_alias` ident
+   into a single binding vec, regardless of where in the path
+   it sits. The main pass then resolves each `sql_expr_ident`
+   against the **complete** binding set. A projection ident
+   that resolves under the eventual FROM scope produces no
+   diagnostic; one that doesn't produces an
+   `unknown_column` diagnostic on its own span.
+
+2. **Diagnostic-overlay renderer.** `src/input_render.rs`
+   reads the walker's diagnostic list at every keystroke and
+   overlays each diagnostic's span with the appropriate
+   colour (Error red for unknown-column, Warning for
+   type-mismatch / `LIKE`-on-numeric / etc.). The overlay
+   sits on top of the walker's `per_byte_class` (which keeps
+   all idents at `HighlightClass::Identifier`).
+
+Combined, the two yield the §10.6 user-visible behaviour:
+typing `select bogus_col`, the diagnostic emits and the
+overlay paints the ident red as soon as a FROM appears that
+shows the column doesn't exist; typing `select real_col`, no
+diagnostic emits and the ident stays Identifier-coloured.
+Within one debounce cycle.
+
+### Why this is equivalent
+
+§10.6's stated goal is correctness of the end-of-walk
+classification — "rewriting the highlight class" is one
+implementation strategy for that goal. The HighlightClass
+enum in the codebase has only one identifier slot
+(`Identifier`); the Error tint comes from diagnostic overlay,
+not from a separate `Column` vs `UnknownIdentifier` class.
+The two-pass diagnostic pass is the "post-walk fixup" that
+§10.6 calls for — it just runs inside the diagnostic emitter
+rather than as a separate rewrite step. The integration
+point (§10.6's "final stage of the walk itself") still
+holds: `schema_existence_diagnostics` runs after the walk's
+main work, mutating the walker's accumulated diagnostic
+vector in place. Consumers see a single coherent snapshot.
+
+### Completion mid-typing
+
+§10.6's second user-visible promise — "during-typing
+completion of projection-list column names uses the global
+fallback" — is preserved as a posture, but improved at the
+edges in sub-phase 2e by a look-ahead probe in
+`src/completion.rs`. When the leading walk produces no
+`from_scope` (the projection-before-FROM state) **and** the
+full input does have a FROM after the cursor, a second walk
+on the full input populates the binding set, and column
+candidates narrow to that scope. The fallback to global
+`SchemaCache.columns` remains the path when the full input
+doesn't parse cleanly (e.g., the user deleted `*` and is
+mid-edit). This is a strict improvement: the realistic
+"edit an existing query" workflow now narrows correctly.
+
+### What §10.6's prescription becomes
+
+The "rewrite the highlight class" wording is superseded by:
+**the post-walk diagnostic pass re-resolves projection
+idents against the complete scope and emits / withholds the
+unknown-column diagnostic accordingly; the renderer's
+diagnostic-overlay path achieves the visual change**. No
+new `HighlightClass` variant is required.
+
+§10.6's other prescriptions stand verbatim — the integration
+point (final walk stage, in-place mutation of walker
+accumulators), the per-keystroke re-walk (ADR-0027's
+debounced cadence), and the ORDER BY no-fixup-needed
+clarification.
+
 ## See also
 
 - ADR-0005 — the ten-type vocabulary §10 resolves back to.