fix(generator): let expr win over pattern and guard empty filenames

[jackchuka]

Follow-up to #84 — two cases where generate still emits a file that fails its own check.

1. Inverted precedence vs. the matcher

resolveHeadingText resolved the filename only when Pattern == "" && Literal == "" && Expr != "", but vast.PatternMatcher.MatchesHeading checks Expr first and ignores Pattern/Literal when it is set. Nothing rejects a heading carrying both keys, so:

structure:
  - heading:
      pattern: "^# .+$"
      expr: "filename == heading"

$ mdschema generate s.yml -o out/Bar.md
$ head -1 out/Bar.md
# ^# .+$
$ mdschema check --schema s.yml out/Bar.md
✗ 1:3 [structure] Heading "# ^# .+$" does not match expression "filename == heading"

Now gated on hp.Expr != "" && outputPath != "", mirroring MatchesHeading. Output is # Bar, check passes.

2. Missing filename == "" guard

matchesHeadingExpr returns early when the extracted filename is empty; resolveHeadingText did not. For an output path whose base is only an extension (-o out/.md), filename == heading evaluated "" == "" → true and a bare # heading was written — one that check time, with the guard in place, can never accept. The generator now falls back to the expression text for that case.

Tests

• docs/.md row added to TestGenerateExprHeading
• new TestGenerateExprWinsOverPatternAndLiteral

go build ./... and go test ./... pass.