ADR-0017 implementation: per-cell type-change with override flags

Replaces the placeholder "trust STRICT" body of do_change_column_type
with the per-cell transformer matrix from ADR-0017. Adds:

- src/type_change.rs: CellOutcome { Clean / Lossy / Incompatible }
  + transform_cell + static_refusal covering every matrix pair
  from §3 (54 unit tests).
- --force-conversion and --dont-convert flags on `change column`
  (mutually exclusive at parse time per §5).
- Refined PK rule (§4.1): refused only when the column has an
  inbound FK and fk_target_type would change. Outbound-FK columns
  still refused outright (§4.2). PK / shortid uniqueness checked
  post-transformation (§4.3).
- Bordered diagnostic tables (lossy / incompatible / collision)
  via the pretty-table renderer (§7) — uses ADR-0016's primitives.
- [client-side] success note (§6) when any cell was rewritten.
- Friendly wrapper for engine-level errors under --dont-convert
  so no engine vocabulary leaks (ADR-0002 user-facing posture).

ADR-0017 §3 + §7 amended in place (with user sign-off): serial->int
added explicitly to the always-clean matrix, and diagnostic rows
identify themselves by PK value(s) rather than positional indices
(SQLite returns rows unordered without ORDER BY, so positional
"row 5" is unaddressable).

Tests: 449 -> 517 (+68). Clippy clean with nursery lints.

src/dsl/value.rs

@@ -208,7 +208,7 @@ impl Value {
     }
 }
 
-fn validate_date(s: &str) -> Result<(), String> {
+pub(crate) fn validate_date(s: &str) -> Result<(), String> {
     // Expect YYYY-MM-DD: 10 chars, two dashes at fixed positions.
     let bytes = s.as_bytes();
     if bytes.len() != 10 || bytes[4] != b'-' || bytes[7] != b'-' {
@@ -231,7 +231,7 @@ fn validate_date(s: &str) -> Result<(), String> {
     Ok(())
 }
 
-fn validate_datetime(s: &str) -> Result<(), String> {
+pub(crate) fn validate_datetime(s: &str) -> Result<(), String> {
     // Minimum: YYYY-MM-DDTHH:MM:SS = 19 chars. Allow optional
     // fractional seconds (.fff) and optional Z or ±HH:MM offset.
     if s.len() < 19 {