docs: teach table checks (#347)

[0x054]

Summary

• Add Table checks section to the models guide: Check(suffix, predicate) in __ferro_checks__, lambda style (I-8), live name ck_<table>_<suffix>, and CheckViolationError on violation — with Assignment/Annotated tabs sharing the same ClassVar (I-7).
• Extend the migrations capability matrix with table-check and column db_check reconcile rows (add / rebuild / drop / leftover warning; SQLite create-inline vs reconcile skip per ADR-0013/0014).
• Update AGENTS.md I-1 item 8 with ck_<table>_<suffix> table-check naming alongside column checks.
• Add runnable docs/examples/table_checks.py + _annotated.py companions; note CheckViolationError on the exceptions API page.

Test plan

• uv run pytest tests/test_docs_examples.py
• Example scripts run end-to-end via docs-examples harness

Issue closure

Closes #347

• Issue status updates complete before merge (does not close parent feat PRD: table-level CHECK constraints (multi-column), auto-migrated #339)

Made with Cursor

[0x054]

Gate: clear

Read the full docs diff. AC for #347 is met.

• Models guide: Check + __ferro_checks__, lambda pair, live ck_pair_at_most_one_side, violation → CheckViolationError
• I-7 tabs share an identical ClassVar; field declarations are tabbed
• I-8 lowercase-singular lambda
• Migrations matrix: create-inline, add/rebuild/drop, leftover warning, SQLite skip
• AGENTS.md I-1 item 8: ck_<table>_<suffix> beside ck_<table>_<col>
• Runnable docs/examples/table_checks.py + _annotated.py

Nits fixed on the PR: dialect paragraph now states the locked ADR-0016 contract (same as the migrate matrix documenting the ladder before sibling slices land); I-1 column-check FFI named _ddl_check_constraint_name.

Merging into feat/table-checks once CI is green. Does not close parent #339.