Docs cleanup + internal-note untracking + MATLAB-mirrored decoding refactor

[iahncajigas]

Summary

This PR removes public-facing porting/parity narrative while preserving the tooling and gate infrastructure, and refactors decoding code to keep a MATLAB-mirrored readable structure.

Commits

• c62c01b Docs: remove porting narrative from README
• 131e3d9 Chore: untrack internal porting notes
• 2662d97 Refactor: restore MATLAB-mirrored structure for DecodingAlgorithms (no numeric changes)

What changed

1) User-facing docs cleanup

• Rewrote README.md as user-facing documentation only:
  • install
  • nstat_install setup
  • quickstart
  • examples/docs links
  • generic developer commands
• Removed references to porting/parity sprints and internal parity narrative.

2) Internal note untracking from public repo

Removed tracked narrative docs from the repository tree and added ignore rules:

• CLEANROOM_POLICY.md
• PARITY_SPEC.md
• CANONICAL_VALIDATION_ARTIFACTS.md
• DISCREPANCIES.md
• parity/CYCLE_VALIDATION_CHECKLIST.md

These can still be maintained internally outside the public repository.

3) MATLAB-mirrored class structure refactor

Updated DecodingAlgorithms in src/nstat/compat/matlab/__init__.py:

• Added explicit MATLAB-mirrored spec helpers for:
  • coefficient draws
  • per-draw rate evaluation
  • probability matrix computation
• Isolated vectorized fast helpers in separate methods.
• Kept readable top-level MATLAB-style staging in _compute_spike_rate_cis_matlab with block comments.
• No numeric behavior changes intended.

Verification

Executed locally:

• pytest -q
• python tools/parity/build_numeric_drift_report.py ... --fail-on-violation
• python tools/parity/check_functional_parity_progress.py ...
• gate-mode validation PDF generation (--parity-mode gate)
• image-mode parity generation + SSIM gate (Failed pages: 0)
• performance-parity check (regression_fail=0)