fix(backtest): reject internal calendar gaps in daily research inputs

[Pigbibi]

Scope

• F3 / RA-10: reject missing calendar days inside the supplied daily panel window before adjacent opens are treated as one-day returns.
• Do not require dates outside the input range, invent prices, or reject gaps limited to unselected symbols while the overall calendar remains complete.
• Exactly src/backtest.py and its existing accounting test file. No dependency/lock edits, provider calls, real replay, publishing, or historical artifact replacement.

Synthetic validation

• Base accounting tests: 24 passed / 18 subtests.
• RED: 2 failed subtests for internal Sunday and Tuesday gaps.
• GREEN accounting + orchestrator: 30 passed / 20 subtests.
• Full UTC-aligned isolated tests: 130 passed / 66 subtests, with env-i and network denied.
• Full Ruff, pip check (50 packages), diff/scope and added-line credential-pattern checks passed.

Existing environment limitations

• Stale uv.lock contains older Firestore/joblib than current pyproject and CI requirements-lock.txt; no lock files were changed. The isolated environment fully resolved current CI requirements, with QPK 7f140f07 and all declared dependencies checked. Missing existing packages were downloaded only for installation; tests denied network.
• First full local-CST run: 129 passed / 66 subtests and one unchanged exporter test failure. Its local-now fixture disagrees with the unchanged UTC cutoff around midnight. Running with CI-equivalent TZ=UTC passed all 130 tests; no exporter/test workaround was committed.
• This is an offline input-quality correction, not new promotion or performance evidence.