feat(lang): rename schedule keyword from extends to modifies

Changed the schedule composition keyword from "extends" to "modifies"
to better reflect the semantic meaning of schedule inheritance. When
a schedule modifies another, it inherits base blocks and can override
them by name or add new blocks.

This is a breaking change for all existing Storybook files that use
schedule composition. The migration is a simple find-and-replace:
  schedule X extends Y → schedule X modifies Y

Changes include:
- Grammar: Updated tree-sitter grammar and lexer token
- Parser: Updated lalrpop parser and AST field names
- Documentation: Updated all reference docs, tutorials, and specs
- Examples: Updated baker-family example schedules
- Tests: Updated all test cases and corpus files
- Testing: Added type system keywords to prop_tests exclusion list
- Tooling: Added xtask for workspace cleanup
- Version: Bumped to v0.3.1 (skipping v0.3.0)
- Spec: Created SBIR v0.3.1 spec documenting the change

BREAKING CHANGE: The "extends" keyword for schedules has been
replaced with "modifies". Update all schedule declarations.

src/syntax/parser.rs

@@ -1,5 +1,5 @@
 // auto-generated: "lalrpop 0.21.0"
-// sha3: 743aa9a8d35318fbf553927304781732c69eaf9c6e511c3951d24e24d2d8d1e8
+// sha3: b4edee3687f9fcc3202af2ee2aea58c7d41ed2aa394ce46e045436e20a36760d
 use crate::syntax::{
     ast::*,
     lexer::Token,
@@ -2761,7 +2761,7 @@ mod __parse__File {
         r###""schedules""###,
         r###""tree""###,
         r###""priority""###,
-        r###""extends""###,
+        r###""modifies""###,
         r###""override""###,
         r###""recurrence""###,
         r###""season""###,
@@ -2976,7 +2976,7 @@ mod __parse__File {
             Token::Schedules if true => Some(36),
             Token::Tree if true => Some(37),
             Token::Priority if true => Some(38),
-            Token::Extends if true => Some(39),
+            Token::Modifies if true => Some(39),
             Token::Override if true => Some(40),
             Token::Recurrence if true => Some(41),
             Token::Season if true => Some(42),
@@ -12251,7 +12251,7 @@ mod __parse__File {
         _: core::marker::PhantomData<()>,
     ) -> (usize, usize)
     {
-        // Schedule = "schedule", Ident, "extends", Ident, "{", ScheduleBody, "}" => ActionFn(434);
+        // Schedule = "schedule", Ident, "modifies", Ident, "{", ScheduleBody, "}" => ActionFn(434);
         assert!(__symbols.len() >= 7);
         let __sym6 = __pop_Variant0(__symbols);
         let __sym5 = __pop_Variant75(__symbols);
@@ -14856,7 +14856,7 @@ fn __action77(
 ) -> Schedule {
     Schedule {
         name,
-        extends: None,
+        modifies: None,
         fields: body.0,
         blocks: body.1,
         recurrences: body.2,
@@ -14886,7 +14886,7 @@ fn __action78(
 ) -> Schedule {
     Schedule {
         name,
-        extends: Some(base),
+        modifies: Some(base),
         fields: body.0,
         blocks: body.1,
         recurrences: body.2,