test(headless): cover tab-indented formula prefix detection

[gregkonush]

Outcome

Add a parser/inspection test that proves formula-prefix detection treats tab-indented formulas the same way it treats space-indented formulas.

Likely files

• packages/headless/src/__tests__/work-paper-sheet-inspection.test.ts
• packages/headless/src/work-paper-sheet-inspection.ts

Suggested approach

Extend the existing detects formula prefixes and compares sheet names test. It already covers =A1, =A1, and a non-formula string. Add assertions for \t=A1 and a non-formula tabbed text cell such as \tA1.

If the current implementation already passes, this is just coverage. If it fails, keep the implementation change minimal and local to formula-prefix detection.

Acceptance proof

Run:

pnpm exec vitest --run packages/headless/src/__tests__/work-paper-sheet-inspection.test.ts

The proof is a passing test with both tab-indented formula and tab-indented non-formula examples.

Scope check

• The task can be reviewed in one sitting.
• The task has one clear validation command or artifact.
• The task does not require broad architecture knowledge.
• The task should stay open for a new contributor unless it is urgent.

Notes

This is a good first patch because it is a narrow expectation with one file, one helper, and one test command.