PERF: Reduce Allocations In Cache Key Builder

[MrEthical07]

Background:
Cache key generation happens on each cacheable read request and is currently string-heavy.

Problem:
internal/core/cache/manager.go key construction uses multiple allocations for canonical value assembly and hashing path, which is a hot cost under load.

Scope:

• Optimize key construction path for lower allocations.
• Preserve deterministic key behavior and cache safety guarantees.
• Expand tests and benchmarks in internal/core/cache/manager_test.go and internal/core/policy/benchmark_test.go.

Tasks:

• Refactor canonical key assembly to reduce temporary allocations.
• Profile and optimize normalized dimension processing.
• Add focused benchmark for BuildReadKey variants.
• Validate no key drift for existing route and vary configurations.

Acceptance Criteria:

• Lower allocs/op and ns/op on key generation benchmarks.
• No cache key correctness regressions for existing configurations.
• Existing cache safety rules remain intact.