feat: add RateLimitParser, metadata validator, memory profiler, and enricher cache

[Ebuka321]

Changes

Closes #746 — Parse Retry-After header with fractional seconds

• Created src/throttle/RateLimitParser.ts with parseRetryAfter(value: string): number | null
• Uses parseFloat for fractional seconds (e.g., "1.5" → 1500 ms)
• Handles HTTP-date format values converted to ms delay from now
• Returns null for unparseable values instead of NaN

Closes #742 — Enforce maximum key length for custom metadata fields

• Created src/validators/invoiceMetadataValidator.ts
• Exports MAX_METADATA_KEY_LENGTH = 64
• validateMetadataKeys() rejects keys longer than 64 characters with descriptive errors
• Validates both top-level and nested custom keys

Closes #739 — Add heap snapshot export to memoryProfiler

• Created src/memoryProfiler.ts with MemoryProfiler class
• exportHeapSnapshot(outputPath: string): Promise<string> writes .heapsnapshot files via v8.writeHeapSnapshot
• Throws ProfilerNotInitializedError if profiler not initialized
• Includes init(), snapshot(), getSnapshots(), reset() methods

Closes #737 — Cache enriched metadata with configurable TTL

• Created src/enricher.ts with EnricherCache<T> class
• getOrFetch(key, fetch) caches results with configurable ttlMs (default 60,000 ms)
• clearCache() empties the cache on demand
• Uses Date.now() for expiry checking — no external cache libraries

Verification

• npm run lint passes
• npm run test (32 tests) passes
• New test file test/modules.test.ts adds 30 tests covering all 4 modules
• Total: 62 tests passing

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