feat(lang): complete extends to modifies keyword migration

This commit completes the migration started in the previous commit,
updating all remaining files:

- Lexer: Changed token from Extends to Modifies
- Parser: Updated lalrpop grammar rules and AST field names
- AST: Renamed Schedule.extends field to modifies
- Grammar: Updated tree-sitter grammar.js
- Tree-sitter: Regenerated parser.c and node-types.json
- Examples: Updated baker-family work schedules
- Tests: Updated schedule composition tests and corpus
- Docs: Updated all reference documentation and tutorials
- Validation: Updated error messages and validation logic
- Package: Bumped version to 0.3.1 in all package manifests

All 554 tests pass.

proptest-regressions/syntax/prop_tests.txt

@@ -8,3 +8,4 @@ cc 8ac445fa78ef3f5ec7fb7d096cbe589988a9478352f82cdac195f5cea57ec47a # shrinks to
 cc 739a6de85e6f514f93fc2d077e929658b31c65294dd44b192972ed882a42171a # shrinks to name = "A", tag = "in", content = ""
 cc 2649e200eb6e916c605196497e1ef5bca64d982d731f71d519c6048671e52ebd # shrinks to name = "if", state_name = "a", target = "A", val = true
 cc 4ab46dfeb431c0c3fe0e894c969f971fd35ae15eec23e51fe170d925c1889526 # shrinks to keyword = "enum"
+cc f205a6ec536bd9dffbf45b238e04090a0095d4609ad9394f0120735c14d82918 # shrinks to ident = "any", val = 0.0