Merge branch 'rate-limit/single-ttl-read' into 'master'

    App[Application code] -->|"check(identifier)"| Limiter

    Limiter -->|delegates| Evaluator

    Evaluator -->|iterates ordered| Rules[Rule list]

    Evaluator <-->|INCR / TTL / EXPIRE| Redis[(Redis)]

    Evaluator <-->|EVALSHA / GET / TTL| Redis[(Redis)]

    Evaluator -->|emits| Metrics[Prometheus metrics]

    Evaluator -->|returns| Result

    Result --> App

Characteristic values longer than 200 bytes are replaced with a SHA-256

hexdigest to bound key length. Missing or empty characteristic values are

encoded as `_unknown_`. The TTL is set on the first write of each window

(`count == 1`) and is not extended on subsequent INCRs, so the window is a

true fixed window starting at the first request, not a sliding window.

encoded as `_unknown_`. The TTL is set on the first write of each window and

is not extended on subsequent increments, so the window is a true fixed

window starting at the first request, not a sliding window.

A check is a single `EVALSHA` of `INCR_SCRIPT` (or `SADD_SCRIPT` for

`count_distinct` rules). Doing the whole read-modify-write inside Lua means

there is no window between the increment and the `EXPIRE` in which a key

could be left without a TTL.

```mermaid

sequenceDiagram

    participant E as Evaluator

    participant P as Connection pool

    participant R as Redis

    participant L as INCR_SCRIPT (Lua)

    E->>P: pool.with { |conn| ... }

    P-->>E: conn

    E->>R: PIPELINE { INCR key, TTL key }

    R-->>E: [count, ttl]

    alt count == 1 (first write of window)

        E->>R: EXPIRE key period

        R-->>E: 1

        Note over E: ttl returned is -1 here;<br/>build_result falls back to<br/>resolved_period for reset_at.

    else count > 1

        Note over E: TTL is not extended:<br/>fixed window from first write.

    E->>R: EVALSHA INCR_SCRIPT key, [period, cost]

    R->>L: run script

    L->>L: INCRBYFLOAT key cost

    L->>L: TTL key

    alt TTL < 0 (key was missing, or had no expiry)

        L->>L: EXPIRE key period

        Note over L: Returns period as the TTL:<br/>the window starts now.

    else TTL >= 0 (window already running)

        Note over L: TTL is not extended:<br/>fixed window from first write.

    end

    L-->>R: {count, ttl}

    R-->>E: [count, ttl]

    E-->>P: release conn

```

`peek` follows the same shape but uses `GET` instead of `INCR` and never

issues `EXPIRE`. A missing key (`GET → nil`, `TTL → -2`) is reported as

`count = 0` and the window is treated as not-yet-started.

Because `INCRBYFLOAT` preserves an existing key's TTL, and creates a missing

key with no expiry, the TTL only needs reading once — after the increment.

That single read distinguishes both cases the script must handle, so there is

no reason to read it again before mutating.

`peek` does not use a script: it pipelines `GET` + `TTL` (or `SCARD` + `TTL`)

and never issues `EXPIRE`, so it cannot start or extend a window. A missing

key (`GET → nil`, `TTL → -2`) is reported as `count = 0`, and `build_result`

falls back to the rule's period for `reset_at` since there is no Redis-side

window to read.

## Result

      # Redis treats the result as a no-op on the stored value while

      # still observing the post-state count and TTL we return.

      #

      # ttl_before < 0 covers TTL=-2 (key missing) and TTL=-1 (no expiry).

      # ttl_after < 0 covers TTL=-2 (key missing) and TTL=-1 (no expiry).

      # The -1 case shouldn't arise with the atomic script, but self-healing

      # recovers keys left without TTL by any prior bug.

      INCR_SCRIPT = Labkit::Redis::Script.new(<<~LUA)

        local ttl = ARGV[1]

        local cost = tonumber(ARGV[2])

        local ttl_before = redis.call('TTL', KEYS[1])

        local count = redis.call('INCRBYFLOAT', KEYS[1], cost)

        if ttl_before < 0 then

        local ttl_after = redis.call('TTL', KEYS[1])

        if ttl_after < 0 then

          redis.call('EXPIRE', KEYS[1], ttl)

          ttl_after = tonumber(ttl)

        end

        return {count, redis.call('TTL', KEYS[1])}

        return {count, ttl_after}

      LUA

      # Atomic SADD + SCARD + conditional EXPIRE. SET-cardinality counterpart

      # of INCR_SCRIPT; same shape (read TTL, mutate, set TTL when missing,

      # of INCR_SCRIPT; same shape (mutate, read TTL, set TTL when missing,

      # return post-state {count, TTL}). count is SCARD, not the SADD return.

      #

      # ttl_before < 0 covers TTL=-2 (key missing) and TTL=-1 (no expiry),

      # ttl_after < 0 covers TTL=-2 (key missing) and TTL=-1 (no expiry),

      # so this also self-heals orphan keys left without TTL.

      SADD_SCRIPT = Labkit::Redis::Script.new(<<~LUA)

        local ttl = ARGV[1]

        local member = ARGV[2]

        local ttl_before = redis.call('TTL', KEYS[1])

        redis.call('SADD', KEYS[1], member)

        local count = redis.call('SCARD', KEYS[1])

        if ttl_before < 0 then

        local ttl_after = redis.call('TTL', KEYS[1])

        if ttl_after < 0 then

          redis.call('EXPIRE', KEYS[1], ttl)

          ttl_after = tonumber(ttl)

        end

        return {count, redis.call('TTL', KEYS[1])}

        return {count, ttl_after}

      LUA

      def initialize(name:, rules:, redis:, logger:)

      end

      # Atomically increments the counter by `cost`, sets the TTL on first

      # write, and reads back the post-increment TTL, all in one Redis

      # write, and returns the window's remaining TTL, all in one Redis

      # operation via Lua. See INCR_SCRIPT for the script body.

      #

      # count is parsed as Float because INCRBYFLOAT returns a string-encoded