Enforce the coverage level already achieved

[EONRaider]

Target: 1.3.1 (Tier 0 — hygiene)

CI runs pytest --cov=netprotocols --cov-report=term but nothing fails
on a coverage drop. [tool.coverage.report] sets only
show_missing = true; there is no fail_under anywhere in
pyproject.toml or .github/.

Measured coverage today is 99% — 1,441 statements, 3 missed, the
misses being packet.py:35 (__repr__) and packet.py:39 (the
__eq__ NotImplemented branch). Every other module is at 100%.

The bar has already been cleared; it just isn't held.

What to do

Add fail_under to [tool.coverage.report] (or --cov-fail-under to
the CI invocation) at a level that locks in current coverage without
being brittle — 95 leaves headroom, 98 is defensible given the margin.

Acceptance criteria

• A PR that meaningfully drops coverage fails CI.
• The current tree passes without modification.