ADR-0018 implementation: auto-fill contracts for serial and shortid

Generalises serial and shortid beyond their previous restricted
forms:

- `serial` is no longer restricted to single-column PK. Non-PK
  serial columns get an emitted UNIQUE constraint and use
  application-side MAX(col)+1 at INSERT time (rowid alias still
  drives the PK case for free; per ADR-0010 worker-thread
  serialisation, the read-then-insert sequence is safe).
- `shortid` columns auto-fill existing null cells when the
  column is materialised — `add column T: x (shortid)` on a
  non-empty table no longer leaves rows in a not-really-valid
  NULL state.
- `int -> serial` joins the type-change matrix as always-clean
  identity (closes the asymmetry vs `text -> shortid`); other
  sources are refused with a route-via-int hint.
- `change column T: x (serial|shortid)` fills null source
  cells with sequence / generated values in the same rebuild
  transaction.

Internal infrastructure:

- ReadColumn gains `unique: bool`; read_schema detects single-
  column UNIQUE indexes via pragma_index_list /
  pragma_index_info; schema_to_ddl emits inline UNIQUE for
  non-PK columns.
- ColumnSchema (persistence) gains `unique: bool` so the flag
  survives YAML round-trip and rebuild-from-text reconstructs
  it faithfully — preserves the "serial -> int leaves UNIQUE
  in place" promise across save/load cycles.
- ChangeColumnTypeResult.client_side now carries `auto_filled`
  + `auto_fill_kind` alongside `transformed` + `lossy`; the
  app handler renders separate note lines when both apply.
- AddColumnResult is a new return type carrying pre-rendered
  [client-side] note lines for the auto-fill paths.

Tests: 519 -> 534 (+15). Clippy clean.

docs/adr/README.md

@@ -23,3 +23,4 @@ This directory contains the project's ADRs, recorded per
 - [ADR-0015 — Project storage runtime](0015-project-storage-runtime.md)
 - [ADR-0016 — Pretty table rendering for data and structure views](0016-pretty-table-rendering.md)
 - [ADR-0017 — Column type-change compatibility](0017-column-type-change-compatibility.md)
+- [ADR-0018 — Auto-fill contracts for `serial` and `shortid` columns](0018-auto-fill-contracts-for-serial-and-shortid.md)