docs: add REVIEW.md

[vincent-k2026]

Adds a REVIEW.md with salt-specific review checks, supplementing the centralized baseline rubric:

• Serde round-trips must be exercised by a test (no true-by-construction assertions that pass on a broken encoder).
• No dead configuration — a documented, set-but-never-read manifest key is a silent no-op for operators.
• Documented local flows must run on stock macOS bash 3.2 (no mapfile; quote empty-array expansions).

The centralized pr-review action reads this file automatically.

[github-actions]

Mutation testing - PASS

Nothing to test: no viable mutants were generated.

[github-actions]

Performance Benchmark Comparison

Compared 5 benchmark(s) against the latest main baseline.

Detailed Comparison

Benchmark	Baseline Throughput (Kelem/s)	New Throughput (Kelem/s)	Change
update 10000 KVs/1 threads	75.69	75.31	-0.50%
update 10000 KVs/2 threads	140.86	142.81	+1.38%
update 10000 KVs/4 threads	263.83	264.56	+0.28%
update 10000 KVs/8 threads	444.87	445.04	+0.04%
update 10000 KVs/16 threads	603.09	601.20	-0.31%

[flyq]

1. The bash rule prescribes the wrong fix — and the review bot will apply it literally (REVIEW.md:12)

"always quote empty-array expansions (an unguarded "${arr[@]}" throws unbound variable under set -u)"

• Quoting is not the remedy. On bash 3.2–4.3, an empty "${arr[@]}" trips set -u whether or not it's quoted (bash 4.4 fixed this). The rule is internally inconsistent: the failing example it shows is already quoted.
• The correct fix is the guard idiom, which this repo already uses and names correctly — scripts/mutation_test.sh:92 says "the nounset-safe ${arr[@]+...} idiom" and line 110 uses ${exclude_args[@]+"${exclude_args[@]}"}.
• This is worse than a normal doc nit because the file is machine-consumed guidance: a reviewer bot told "always quote" would approve "${arr[@]}" — exactly the pattern that breaks on stock macOS — and could flag the correct guard as redundant.

Suggested replacement: "…no mapfile, and guard possibly-empty array expansions with the nounset-safe ${arr[@]+"${arr[@]}"} idiom — under set -u, bash ≤ 4.3 treats an empty "${arr[@]}" as unbound even when quoted (see scripts/mutation_test.sh)."

[flyq]

2. Minor: "just/shell entry points" (REVIEW.md:12) — there is no justfile anywhere in this repo; local flows are plain scripts (scripts/mutation_test.sh, scripts/mutation_gate.py). Fine as future-proofing, but if it's a leftover from another repo's REVIEW.md, drop "just/" so reviewers don't hunt for recipes that don't exist.