feat: add safe login profile research tool

[subzeroid]

Summary

• Add an opt-in maintainer script for comparing reused and fresh device profiles across controlled login accounts, with separate-account and crossover pairing modes.
• Bound all experiments to ten sequential attempts, enforce a minimum cooldown and per-login timeout, require a verified HTTPS account pool without redirects, and validate every account record before use.
• Keep results pseudonymous and secret-safe through per-run keyed profile digests, sanitized error classes, muted client/request loggers, session cleanup, and owner-only JSONL output protected against symlink and path-swap attacks.
• Add 56 network-free regressions and document the controlled workflow in the development guide.

Safety

• Requires both IG_RUN_LOGIN_MATRIX=1 and TEST_ACCOUNTS_URL; GitHub Actions never invokes the script.
• Never records usernames, passwords, TOTP data, sessions, proxy values, pool URLs, exception messages, or response bodies; generated login-matrix*.jsonl files are ignored by Git.
• No live Instagram experiment was run for this PR. The remaining real pool-to-login flow is intentionally manual-only because repeated logins can trigger checkpoints or restrictions.

Test Coverage

• AI-assisted branch audit: 28 of 30 paths covered (93%).
• Covered paths include configuration bounds, fetch and payload failures, separate/crossover planning, stable/fresh isolation, TOTP, success/false/error/timeout/cancellation outcomes, logger suppression, session cleanup, secure output permissions and races, sequential orchestration, and continuation after an error result.
• Known gaps: injected write()/flush() failure cleanup and the manual-only live login flow.

Verification

• uv run --extra test python -m pytest -q tests/regression/test_research_login_matrix.py — 56 passed.
• uv run --extra test python -m pytest -q tests/regression — 655 passed, 3 skipped, 35 subtests passed.
• uv run --extra test pre-commit run --all-files — passed.
• uv run --extra test bandit -q -c pyproject.toml -r aiograpi scripts/research_login_matrix.py — passed.
• uv run --extra test mkdocs build --strict — passed.
• Independent final review: no Critical, Important, or Minor findings.

Documentation

• docs/development-guide.md documents the controlled login-profile research command, modes, safeguards, limits, sanitized output, failure behavior, and non-CI status.
• Maintainer entry points already link to the development guide; no README, migration, diagram, changelog, or version update is needed.
• Strict MkDocs build passes; documentation debt: none.