"\(x)" in text, test blocks with assert and finch test, a build cache, the chain of calls on runtime errors

- "x is \(x)", "\(speed, 2)" (two digits after the point), also in """ text
- test "name" { ... } blocks; assert(cond) and assert(cond, why); finch test runs each on its own
- the program's object and its C files (link "x.c") are kept in the cache until they or their headers change
- runtime errors show the calls that led there (finch run, finch test, -g, -O0, --trace)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016GzNP6r8dg6WDxUVZy8wjL

src/ast.fch

@@ -123,4 +123,5 @@ struct Program {
     []Node enums
     []Node fns
     []Node consts   // const NAME = value
+    []str tests     // the functions made from test "..." { } blocks, in order
 }