Attach contracts to definitions

Contracts now live directly on definitions via @ / =@ annotations and
travel automatically with exported values.

- Remove !export from lexer/parser/AST/evaluator/manifest/resolver and CLI.
- Simplify workspace module export logic: export all top-level local
  definitions by default.
- Update Frontend.ContractDesugar:
  - Named binder annotations (x@nat?) expand to per-argument withContract.
  - Phantom annotations (@nat?) expand to a local raw helper plus a wrapper,
    keeping fixed points shared and only depending on withContract.
- Merge lib/guardedBase.tri into lib/base.tri and annotate partial/sensitive
  base functions: head, tail, last, add, sub, mul, div, mod, pow, min,
  max, length, sum, product.
- Add check contract helper to lib/base.tri.
- Update demos/contractBasics.tri and README to reflect @/=@-only design.
- Update test suite: remove guardedBase import, replace explicit !export
  test with a test verifying that contract annotations on an exported
  definition are enforced on import.
- Fix remaining base.tri definitions (div/mod/pow) to stay point-free.

README.md

@@ -63,8 +63,9 @@ tricu eval --format decode program.tri
 tricu eval --output result.txt program.tri
 ```
 
-Annotations are parsed but currently ignored at runtime; the contract layer
-is not yet wired into evaluation or workspace module auto-builds.
+Contract annotations (`@` and `=@`) attach guards directly to definitions.
+When a workspace module is built, those guarded definitions become the
+exported values, so contracts travel with imports automatically.
 
 Compile/import/export Arboricx bundles: