Add language-agnostic conformance suite for credit lease semantics

[bpapillon]

Precursor to SCH-6531 (lease support in other SDKs): extracts the credit lease/reservation semantics from this SDK — the reference implementation — into a language-agnostic conformance suite, so the upcoming Python/Go ports can prove "implements the same semantics" instead of approximating it.

• conformance/SPEC.md — the semantics spec (state machine, store-op contracts, check/track flows, config knobs, bounded-leak contract table, and the invariants that can't be vectors), written for implementing from scratch without reading the Node source.
• conformance/vectors/*.json — 48 deterministic test vectors (plain JSON, snake_case, virtual clock): reservation lifecycle 7, lease lifecycle 9, expiry 4, crash windows 5, lease manager 10, check flow 10, track/settle 3.
• tests/conformance/runner.test.ts — runs every vector against both store backends (in-memory + the existing fake-redis Lua interpreter): 95 passing runs. The runner is the only language-specific piece a port reimplements.

Behaviors worth calling out (vectors follow the code):

• Over-use on settle is asymmetric: the local consume clamps to credits_reserved, but the Track event bills the unclamped actual quantity (server is source of truth).
• fail-open is not blanket allow — the engine re-runs with a substituted max balance, so an unentitled company stays denied while the lease backend is down.
• replace keeps a live lease even under a different lease id; the losing acquire releases its redundant lease only when the server minted a different id.
• Store extend reconciles to the server's absolute total (lower/equal totals are no-ops) and expiry only moves forward; both extend and refund are pinned to the originating lease id and silently dropped if a successor holds the slot.
• Crash windows are direction-locked: the debit/claim is durable first, so crashes leak bounded, lease-expiry-reclaimed credits and can never double-spend or double-refund.

Not capturable as vectors (documented as prose invariants in SPEC.md): single-flight/concurrent-race schedules, Redis server-clock TTL authority and single-key Lua atomicity, NaN guards (JSON can't encode NaN), Track idempotency-key dedupe, the background sweep timer, and offline/unconfigured wrapper degradation.