feat!: implement numeric coercion contract #1996

Implements the numeric coercion ADR in flagd and flagd-core (Go).

• implement lossless numeric coercion in the evaluator (core/pkg/evaluator/json.go): replace the bare int64(val) cast in ResolveIntValue with a lossless check (reject a non-zero fractional part and out-of-accessor-range values with TYPE_MISMATCH); 10.0 -> 10 and lossless widening to Float continue to work
• contract applies uniformly to gRPC, OFREP, and in-process paths (no wire changes)
• bump the flagd-schemas submodule to pick up the safe-integer range constraint. This remains an ordinary schema validation (surfaced as today, i.e. a warning); no new hard load-failure path is added here. Hard-failing on schema violations is a separate future feature.
• bump the flagd-testbed submodule and adopt the new @numeric-coercion scenarios
• update ADR status: draft -> accepted; add the per-language accessor-width table

Accessor width: Go's ResolveIntValue is int64, i.e. Go's Integer accessor is the canonical Long. Go therefore excludes the @int32-bounded testbed scenarios; no separate Long accessor is required in Go core.

Breaking (lossless coercion changes existing truncation behavior, e.g. 3.14 via Integer now returns TYPE_MISMATCH); feat!:, minor bump, call out in release notes.

Parent: #1995