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.

docs/tutorial/07-schedules.md

@@ -130,7 +130,7 @@ schedule MarketSchedule {
 
 Recurrences take priority over regular blocks. On Saturdays, the `MarketDay` blocks replace the regular `work` block.
 
-## Schedule Composition with extends
+## Schedule Composition with modifies
 
 Schedules can extend other schedules, inheriting and overriding blocks:
 
@@ -142,7 +142,7 @@ schedule BaseShopkeeper {
     }
 }
 
-schedule EarlyBaker extends BaseShopkeeper {
+schedule EarlyBaker modifies BaseShopkeeper {
     block open {
         05:00 - 13:00
         action: baking::early_shift
@@ -155,7 +155,7 @@ The `EarlyBaker` schedule overrides the `open` block by name -- same block name,
 You can chain extensions:
 
 ```storybook
-schedule MasterBaker extends EarlyBaker {
+schedule MasterBaker modifies EarlyBaker {
     block open {
         03:00 - 11:00
         action: baking::master_work