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feat: implement global keyword for Python §3 and §4 (#173)

* feat: implement global keyword for Python chapters 3 and 4

Adds full resolver and CSE interpreter support for the `global`
statement added to the Python §3/§4 spec in PR #162.

Resolver changes (resolver.ts):
- scanGlobalDeclarations() recursively finds `global x` declarations
  in a function body without descending into nested functions
- visitFunctionDefStmt pre-declares globally-named variables in the
  outermost environment so chain-walk lookups find them
- resolve() pre-pass skips locally hoisting names declared global
- visitForStmt skips declaring the loop target locally when global

CSE interpreter changes:
- closure.ts: adds globalVariables: Set<string> field
- utils.ts: adds getProgramEnvironment(), pyGetGlobalVariable(), and
  scanForGlobalDeclarations(); scanForAssignments() now accepts a
  globalNames exclusion set so global names stay out of localVariables
- interpreter.ts: Global node handler (no-op); FunctionDef computes
  globalVariables before building the closure; Variable reads from
  programEnvironment for global names; ASSIGNMENT writes to
  programEnvironment for global names

Validator:
- NoGlobalValidator blocks `global` in chapters 1 and 2

Tests (global-keyword.test.ts): 13 tests covering validator
rejection/acceptance, basic write, read-through, repeated mutation,
scope isolation between functions, nested-function bypass of enclosing
scope, global inside an if-branch, late creation of a global name, and
print output.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: recurse into single-node AST children in scanForGlobalDeclarations

The previous implementation only iterated over array-valued properties,
missing single-object children such as the elif node stored in
If.elseBlock. A global declaration inside an elif branch would silently
be treated as a local variable, causing a NameError at runtime.

Now also recurse into any non-array child that... (continued)

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