Dynamic variable parity: $timestamp offsets, $localDatetime and the $random.* family #34

Why

apic's built-ins (internal/runner/vars.go:129-189) cover $uuid, $timestamp, $isoTimestamp, $datetime, $randomInt, $processEnv, $dotenv and $env.X. REST Client files also use offsets ({{$timestamp -1 d}}, {{$datetime iso8601 1 h}}) and {{$localDatetime}}; JetBrains files use $random.integer(1, 100), $random.alphabetic(10), $random.email and friends. Those files currently fail with a missing-variable error under apic.

What

Acceptance criteria:

• $timestamp [offset unit] and $datetime <format> [offset unit] accept the REST Client offset grammar (-1 d, 2 h, 30 m, -10 s, 1 w, 1 M, 1 y).
• $localDatetime [format] formats in the local zone with the same options as $datetime.
• JetBrains: $random.integer(min, max), $random.float(min, max), $random.alphabetic(n), $random.alphanumeric(n), $random.hexadecimal(n), $random.email, $random.uuid. Argument-less forms take the JetBrains defaults.
• $exampleServer is deliberately not added (JetBrains-only concept); $projectRoot resolves to the apic project root since it is cheap and useful for < {{$projectRoot}}/fixtures/x.json.
• The nesting cap of 8 (vars.go:76-78) still applies; the random functions are not nonces and keep the existing //nolint:gosec reasoning.

Where

internal/runner/vars.go; internal/runner/runner_test.go.

Docs

docs/format.md (Built-ins table), docs/cheatsheet.md (Built-in placeholders).

Tests

A table-driven test per function, with a fixed clock injected for the time-based ones; a parse-error test for a bad offset that names the placeholder and the line.