[withrvr]

Phase 2 (feature 6 of 6): Coverage, reconciliation, docs close-out

What this adds

• tests/validate/test_coverage.py: every rule in RULE_REGISTRY fires
  at least once on dataset A, the registry covers exactly E001-E014, and
  the total row count is 17 — the Phase 2 done-when criterion.
• tests/test_reconcile.py: two reconciliation identities proving no
  invoice or payment rupee is lost. Caught and fixed a real bug in the
  original design: a draft equation assumed "overpaid" (payments
  exceeding invoice amount) is nonzero only for the E014 case (INV-1007).
  That's false — INV-1028's negative invoice amount with zero payments
  against it produces max(0, 0 - (-45000)) = 45000 under the same
  formula, since compute_outstanding's floor doesn't distinguish
  "genuine overpayment" from "floored negative amount." Verified directly
  against dataset A (total_overpaid = 67,000, not 22,000) rather than
  assumed, before writing the test.
• Docs close-out: RULES.md reordered into E001-E014 numeric order (was
  in PR-landing order) with a dataset-A row-count cross-check against
  QA_PREP.md Q7; README.md's "Validation performed" section filled in;
  DEVELOPMENT.md status/testing updated; CHANGELOG.md 0.3.0 entry.

Phase 2 is now complete: all 14 rules implemented, 107 tests passing,
RULES.md matching the code exactly.

Assessment requirement served

Closes out "generate an exception report," the never-silently-drop
standing rule, and the validation-approach grading criterion with a
concrete, checkable coverage + reconciliation story.

Docs updated

• README.md
• ARCHITECTURE.md (no behavior change this PR)
• API.md
• RULES.md
• DEVELOPMENT.md
• CHANGELOG.md

Verification

cd backend && uv run pytest app/collections/tests -v
# 107 passed

uv run python -m app.collections.scripts.reference_summary
# still 15 overdue / Rs 1,202,000.00 / West heaviest

Known gaps carried to next phase

• No SQLModel persistence, run_events, or API wiring — Phases 3-4.
• The independently-derived SQL crosscheck (test_sql_crosscheck.py)
  stays deferred to Phase 3+, since it needs persistence to recompute
  from; the reconciliation tests here are internal-consistency checks,
  not that independent method.
• v0.3.0 tag will be attempted on merge; Phase 1 found git push origin <tag> fails with a 403 in this sandbox (no tag/release tool
  available via the GitHub MCP server either) — if it fails again, the
  CHANGELOG entry stands as the release record.

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