What

Adds an ast field to MacroContext that exposes the AST handles for the current macro invocation:

• ast.call — the CallExpression node for this macro invocation. For await fn() macros the inner CallExpression is returned (not the wrapping AwaitExpression). For tagged-template macros, a synthesized CallExpression is provided whose loc/start/end match the original TaggedTemplateExpression.
• ast.program — the Program AST of the enclosing file.

Also:

• Documents the field (and the wider MacroContext surface) in the README, with a $callsite example.
• Adds a fixture (tests/fixtures/ast.js + tests/fixtures/macros/ast.ts) and a type-level test in tests/define.test.ts.

Why

Closes #132. Macro authors previously had no way to get callsite-aware info (line/column, raw call source, surrounding program) without resorting to runtime stack walks or re-parsing the file. Exposing the already-available Babel nodes is essentially free and unlocks patterns like $callsite(), conditional rewrites, and lightweight code analysis at compile time.

Test notes

• New snapshot in tests/__snapshots__/fixtures.test.ts.snap covers the ast.js fixture (line/column/calleeName/callSource/programType all round-trip correctly).
• New type assertions in tests/define.test.ts ensure this.ast.call.type and this.ast.program.type are correctly narrowed.

Follow-ups

• unpluginContext remains marked experimental; ast is intentionally a stable, minimal surface (call + program) so we can extend it later without breaking changes.