ci: Add value-conservation invariant suite for the constant-sum curve

[hunterinvariants]

What this adds

A value-conservation invariant suite (test/CounterHunterInvariant.t.sol) for the constant-sum hook, plus a CI workflow that re-runs it on every PR.

Hand-configured to mirror your Counter.t.sol setUp pattern — the constant-sum design intentionally rejects native v4 liquidity ("No v4 Liquidity allowed"), so the standard Deployers.initPoolAndAddLiquidity route doesn't apply. The suite uses manager.initialize + hook.addLiquidity directly, matching your existing test.

What it checks

Two universal value-conservation properties under invariant fuzzing:

1. invariant_no_free_swap_round_trip — a 0→1→0 round-trip can't return more than it spent. For a 1:1 constant-sum curve, this is the math claim under randomized inputs.
2. invariant_callbacks_reject_non_poolmanager — every hook callback reverts unless the PoolManager is the caller. Catches the missing-onlyPoolManager guard class.

LP-side invariants (LP add/remove can't profit, LP can always withdraw) don't apply to a hook-managed liquidity model and were dropped from this suite.

Verbatim verdict on this hook

Ran 1 test suite in 136.26s -- 2 passed, 0 failed, 0 skipped
HUNTER_MAX_SWAP_RT_DELTA: 0

Across 12,800 randomized 0→1→0 round-trips at 64 runs × 200 depth, the maximum swapper-extractable margin was 0 wei. The constant-sum curve is round-trip-conservative under fuzz at the wei level.

Cost

• Two files (one .t.sol + one workflow yaml).
• ~140 s of CI per PR.
• Opt-in. Deleting the workflow removes the gate.

Honest framing

• Fuzzing, not proof — not a full audit. It checks the value-conservation thesis; it doesn't replace human review of the accounting logic.
• The invariant suite is part of Hunter, a generic regression gate for v4 hooks. Hunter's auto-generator handles standard archetypes; the constant-sum case is a custom-curve archetype where the auto-generator falls back to hand-config — this PR is the worked example.
• MIT, free, no telemetry, no strings.

Feel free to close if not useful — no hard feelings.