Add a codecov.yml so the project coverage status tolerates ordinary drift

[alongd]

What

Adds a codecov.yml (23 lines, no code change):

• coverage.status.project.default.threshold: 1%
• coverage.status.patch.default.target: 80%

Why

ARC has no Codecov configuration at all — .coveragerc configures coverage.py's collection, not
Codecov's status checks. With no config, Codecov falls back to its built-in defaults, under which
the project status is target: auto with a 0% threshold: total coverage may never dip by any
amount relative to the base commit.

The consequence is that nearly every PR adding code covered below the repository average (~67%)
reports a failing check while every test job, CodeQL and the functional suite are green. Observed
on the current open set, all red on codecov/project alone:

PR	status
#653	67.03% (-0.10%) → 67.05% (-0.08%) after adding a genuine unit test
#860, #1016, #1039, #1050, #1054, #1055, #1056, #1057	red on codecov/project, everything else green

On #653, codecov/patch simultaneously reported "Coverage not affected" — the gate that actually
judges whether the PR's new code is tested was already satisfied. A check that fails on
one-tenth of a percentage point trains reviewers to ignore it, which costs us the patch signal too.

What this does and does not change

• Does: lets total coverage drift up to 1% without failing the status.
• Does: keep an explicit patch target, so the coverage of a PR's own new or changed lines
  remains a gating signal. This is the stricter half of the config, not a loosening.
• Does not: touch any test or source file, change .coveragerc, or alter what coverage is
  collected.
• Does not: raise ARC's actual coverage. That is a separate and much larger piece of work.

Note that Codecov re-evaluates statuses on new uploads, so already-open PRs will pick this up on
their next CI run rather than retroactively.

[Copilot]

Copilot review overview

🟢 Approval recommended

The Codecov configuration is valid and matches the stated coverage policy.

Review effort: Balanced
Findings: None

What changed in this PR

Adds Codecov status configuration to tolerate minor overall coverage drift while preserving patch coverage enforcement.

Changes:

• Allows project coverage to decrease by up to 1%.
• Requires 80% coverage for changed lines.
• Configures Codecov’s PR comment layout.

File	Description
codecov.yml	Defines project and patch coverage thresholds and comment layout.

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.07%. Comparing base (a7c2666) to head (ceff564).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1066   +/-   ##
=======================================
  Coverage   67.07%   67.07%           
=======================================
  Files         123      123           
  Lines       43663    43663           
  Branches    11152    11152           
=======================================
+ Hits        29285    29288    +3     
+ Misses      11224    11219    -5     
- Partials     3154     3156    +2     

Flag	Coverage Δ
functionaltests	67.07% <ø> (+<0.01%)	⬆️
unittests	67.07% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 5 files with indirect coverage changes

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