Heuristic JCL/COBOL detection and syntax highlighting in the raw record view

[Tannex]

Implements #21. Stacked on PR #27 (issue-19-table-polish) — merge that first; this PR's own diff is commit d7da6a2.

What

• Detection (internal/cqt/syntax.go): classifies a data set/member's decoded records as JCL, COBOL, or plain.
  • JCL: ≥60% of sampled non-blank lines start with // or /* (min 2).
  • COBOL: scored signals — division headers (strong), PIC/PICTURE clauses, column-7 comment indicators, and a numeric sequence area on most lines (supporting signal only, so flat data files with leading record ids stay plain).
  • Anything below the bar renders exactly as before.
• Caching: the verdict is computed once per loaded page — cached on the workspace keyed by record-cache size, reset alongside rawLongest on refresh/reset. Never per keystroke or per render frame; content type can't change mid-file, so a 120-line prefix sample carries the verdict.
• Highlighting (raw record view only): pure per-line tokenizers using the existing consolePalette, matching the table-polish tones:
  • JCL: comments/delimiters muted, //NAME cyan, operations (JOB/EXEC/DD/…) amber, KEYWORD= parameter keys and quoted literals green, in-stream data plain.
  • COBOL: sequence area muted, column-7 comment lines muted wholesale, keywords/verbs cyan, PIC + picture strings amber, quoted literals green.
• Highlighted lines strip back to their exact input (ansi.Strip(highlight(l)) == l), so the viewport's ANSI-aware ansi.Cut keeps horizontal panning and narrow terminals safe. The cursor row stays untinted so the selected-row style renders uniformly (same rule as the table cell tints).
• Demo mode: DEMO.JCLLIB members serve a realistic JCL job and DEMO.SOURCE.COBOL members a COBOL program, exercising every token class offline (and flowing through the edit-mode text path too).

Screenshots (demo mode, via scripts/tui-shot)

JCL (120×40)	COBOL (120×40)	JCL narrow (70×24)

Decisions

• Highlighting applies to the raw record view only. The editor (bubbles textarea) renders its own content, so reusing the highlighter there is not trivial — noted as a possible follow-up.
• Keyword sets are deliberately partial (common operations/verbs/clauses); a full reserved-word list would add noise, not signal.

Testing

go build ./... && go vet ./... && go test ./... all pass, gofmt clean. New tests cover detection (JCL, COBOL, flat-data and mostly-plain negatives), both highlighters' token classes with content round-trip guarantees, once-per-page cache behavior (content swap without count change is not re-detected; a new page is), raw-view integration with an untinted cursor row, and unchanged plain rendering.

Closes #21

🤖 Generated with Claude Code

[Tannex]

A bit heavy on in-line comments, could indicate the code needs refactoring.

Could use better support for COBOL keywords, but we can do that another time.

LGTM;