Consider per-file Apache-2.0 license headers

[shibaji7]

Background

PR #16 (chore/guardrails) evaluated adding an insert-license pre-commit hook that enforces an Apache-2.0 header block on every .py file. It was skipped with the following reasoning:

• LICENSE at repo root grants Apache-2.0 for the entire tree — the actual legal grant is covered.
• Per-file headers matter only when (a) someone vendors a single file into another codebase, or (b) legal counsel of a corporate contributor requires it, or (c) a JOSS reviewer explicitly asks. None of those apply today.
• Adding headers touches ~50 files with pure visual noise for zero technical value. Beginners will forget the header on new files → CI red → poor first-time experience.

When to act on this

Trigger conditions:

1. A JOSS reviewer asks for per-file headers during review.
2. A corporate contributor's legal counsel requires per-file attribution.
3. Someone forks and vendors a single GeoPulse module into a downstream project (rare for scientific Python).

Estimated cost when we do act

• Add insert-license pre-commit hook in .pre-commit-config.yaml with the standard 3-line Apache-2.0 header.
• Bulk-add headers to every src/geopulse/**/*.py file (~1 PR, ~200 files, mechanically generated).
• Document the requirement in CONTRIBUTING.md as a survival-guide entry.
• Total: ~1 hour.

Priority

Low. Do not action unless one of the trigger conditions above is met.