feat: add rule names as stable counter keys (Stage 1b)

What

Closes gitlab-com/gl-infra/production-engineering#28787 (closed)

Implements Spec 7 / Stage 1b: replaces rule_index in Redis counter keys and structured log payloads with a required name field on Rule.

Rebased onto !272 (merged) (Spec 8 / Limiter redesign). All Spec 7 behaviour now lives in Limiter (which owns the logger in the new architecture).

Why

rule_index (the 0-based array position) is unstable — inserting or removing a rule silently reassigns every other counter to a new key, abandoning in-flight counts mid-window. A name-based key is stable across reorderings and readable in logs and on-call tooling.

Changes

lib/labkit/rate_limit/rule.rb

• name: is now a required keyword on Rule
• Validates type (String/Symbol — always raises), format /\A[a-z0-9_]+\z/, and max length 64 chars (raises in dev/test via Labkit.dev_or_test?; production accepts any name for Limiter to sanitize)
• RULE_NAME_PATTERN / RULE_NAME_MAX_LENGTH defined at RateLimit module level (not inside the Data.define block)

lib/labkit/rate_limit/limiter.rb

• prepare_rules runs at Limiter.new time: sanitizes invalid names (WARN + downcase/replace/truncate), deduplicates by sanitized name (raise in dev/test, drop first-wins in production with WARN)
• Limiter#check emits rate_limit_check WARN when a :block rule is exceeded
• Redis key shape (from !272 (merged)): labkit:rl:{name}:{rule_name}:{char}:{value}

Specs

• spec/labkit/rate_limit/rule_spec.rb: name validation scenarios (type, format, length)
• spec/labkit/rate_limit/limiter_spec.rb: dedup, sanitize, post-sanitize collision, WARN on exceeded :block, no WARN on :log, evaluator reuse
• spec/labkit/rate_limit_spec.rb: reorder-stability (keys accumulate by rule name regardless of array order), no integer-index key segments

Verification

71 examples, 0 failures. Rubocop: no offenses.

Depends on !272 (merged) (Spec 8 / Limiter redesign) — rebased onto rate-limit/stage-1a-limiter-redesign.