[P2] Clipping and measurement helpers

[fissible]

Shared helpers used by layout, scroll container, and text rendering:

• visible range calculation
• text width clipping (respects ANSI escape codes)
• padding/border offset math
• viewport coordinate math

Deliverable: Functions in src/draw.sh or a new src/measure.sh.
Effort: S (1–2h) | Phase: 2