UX: MISSING requirement chips show no reason (need explanatory tooltip/legend)

[vivganes]

Severity: High (umbrella UX fix)
Source: PR #5 review.

Problem
The report renders e2e(checkout) OK / e2e(checkout) MISSING chips. A requirement can be MISSING for several different reasons, but the UI shows the identical red chip for all of them, with no "why". This is the worst kind of UX problem — the user doesn't know they misconfigured something.

Expected
Every MISSING chip should carry a human-readable reason so the user can self-diagnose. Add a tooltip/reason such as:

• 0 results under module "checkout" (config modules: parsers, linker, …)
• results exist under module "payments", not "checkout"
• results exist but are unscoped ("")

Fix

• Add a reason field to the requirements computation (in report_model.py:_requirements and renderers.py:_required_layers).
• Render it as a title= tooltip (and/or visible sub-text) on the chip (renderers.py:649).
• This closes the symptom half of Need online docs #1, add v1 docs #2, feat: machine-readable json output with schema #3, fix: report unlinked tests regardless of @FC- tag prefix #4, feat: support module-scoped e2e requirements #5.

Related: #1 (case), #2 (module validation), #3 (missing config key), #4 (coverage vs requirement), #5 (unscoped).