test: mutation-harden LibMemoryKV overflow boundary coverage

[thedavidmeister]

Group hardened: MemoryKVOverflow boundary (max valid 0xFFFF pointer acceptance)

Scoped adversarial-mutation pass over the sole module src/lib/LibMemoryKV.sol
(get / set / toBytes32Array). Tests-only — git diff src/ is empty, no
source or bytecode change.

The mutation sweep found the suite to be very strong: every mutation across slot
spread, pointer stride, key/value placement, length tracking, allocation, and the
entire toBytes32Array bisect tree was already killed. The one weak spot was
the overflow boundary in set:

if (pointer > 0xFFFF) {
    revert MemoryKVOverflow(pointer);
}

testSetOverflow only drove the free-memory pointer to 0x10000 and asserted a
revert — the over-boundary case. Nothing asserted that an insert landing on the
maximum valid 16-bit pointer 0xFFFF is accepted, so off-by-one mutations of
the boundary survived undetected. This is exactly the line issue
#13 (16→32-bit pointer
rewrite) will touch; pinning the boundary now protects that future change. This PR
does not implement #13.

Mutation matrix (behavior → mutation → killer)

Behavior	Mutation	Pre-PR	Killer
overflow boundary lower edge	pointer > 0xFFFF → >= 0xFFFF	SURVIVED	testSetPointerBoundaryMaxAccepted (new)
overflow boundary lower edge	pointer > 0xFFFF → > 0xFFFE	SURVIVED	testSetPointerBoundaryMaxAccepted / ...BelowMaxAccepted (new)
overflow boundary upper edge	revert payload 0x10000	killed	testSetOverflow + new ...OverflowReverts (exact payload)
get: slot spread mod(...,15)*0x10	*0x10→*0x20, 15→16	killed	existing
get: pointer mask 0xFFFF walk	stride 0x40→0x60	killed	existing
get: key-compare slot mload(ptr)	→mload(ptr+0x20)	killed	existing
get: value read ptr+0x20	→ptr+0x40	killed	existing
get: exists := 1	→0	killed	existing
set: slot spread mod(...,15)	15→16 (get/set sync)	killed	existing
set: walk stride 0x40	→0x60	killed	existing
set: update value slot ptr+0x20	→ptr+0x40	killed	existing
set: insert key/value placement	swap key↔value	killed	existing
set: insert next-ptr startPointer	→0 (chain break)	killed	existing
set: length +2	→+1	killed	existing
set: alloc bump 0x60	→0x40	killed	existing
set: length/pointer write masks	not(shl(...,0xFFFF))→not(0)	killed	existing
set: bitOffset write shl(bitOffset,ptr)	→shl(0,ptr)	killed	existing
toBytes32Array: length shr 0xf0	→shr 0xe0	killed	existing
toBytes32Array: copyFromPtr stride/cursor/key/value	offset mutations	killed	existing
toBytes32Array: bisect leaves (all 15)	force-skip a leaf	killed	testSaturate
toBytes32Array: bisect masks (mask128, shr 0x90)	narrow/shift	killed	existing

New tests

• testSetPointerBoundaryMaxAccepted — insert with free-mem-ptr at exactly
  0xFFFF must NOT revert; asserts the pointer 0xFFFF is encoded into a kv
  slot and length is exactly 2.
• testSetPointerBoundaryBelowMaxAccepted — same for 0xFFFE, guarding the
  boundary from below.
• testSetPointerBoundaryOverflowReverts — 0x10000 reverts with the exact
  MemoryKVOverflow(0x10000) payload.

(The accepted-case tests deliberately assert the kv pointer encoding and
non-revert rather than reading the value back: a node placed at 0xFFFF lives in
low memory and may be clobbered, so the encoding/non-revert are the safe oracles.)

Verification

• forge build: clean (Compiler run successful!; only pre-existing boolean-cst
  lint warnings from the source bisect tree / slow ref impl, unrelated to this PR).
• forge fmt --check: clean.
• Full suite: 28 passed, 0 failed (was 25; +3).
• Each new accepted-case test confirmed to FAIL under both surviving mutations and
  PASS on the restored baseline; source restored (git diff src/ empty).

Remaining gaps checklist

• No standalone test pins MemoryKVOverflow for the update path (updates
  never allocate, so they cannot overflow — currently a non-issue, but worth a
  note if support larger pointers #13 changes allocation).
• Boundary accepted-case asserts pointer encoding, not a full get
  round-trip at 0xFFFF (intentional — 0xFFFF low memory is clobberable).
• get/set hash-sync is covered only indirectly (both mod(...,15)
  mutations are killed); no dedicated test asserting identical slot derivation.
• The 15-slot bisect leaves are killed only by the statistical testSaturate
  fuzz (which deterministically fills all slots via re-hashing); no
  per-leaf deterministic unit test.

🤖 Generated with Claude Code

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Reviewing files that changed from the base of the PR and between 0c2239a and 3b22a94.

📒 Files selected for processing (1)

• test/src/lib/LibMemoryKV.getset.t.sol

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• Create PR with unit tests
• Commit unit tests in branch 2026-06-15-memkv-coverage

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[thedavidmeister]

Reviewed 3b22a94: mutation-validated tests-only coverage for the LibMemoryKV max 0xFFFF pointer-overflow boundary (was untested; >/>= and >0xFFFE mutants survived); source byte-identical. LGTM.