refactor: extract payload ingestion and validation module from HTTP handler (#139)

[jonbaldie]

Closes #139

Summary

Extracts payload streaming, size limits, UTF-8 decoding, JSON parsing, nesting depth limits, and number precision validation from src/handler.ts into a dedicated src/payload.ts module.

Changes

• src/payload.ts: creates deep payload ingestion module exporting readAndValidatePayload, parsePayloadBody, constants (DEFAULT_MAX_PAYLOAD_SIZE, MAX_JSON_DEPTH), and domain errors (PayloadTooLargeError, InvalidPayloadError, JsonNestingTooDeepError, UnsupportedNumberError).
• src/handler.ts: delegates request payload parsing to src/payload.ts and maps errors to HTTP 400 and 413 responses in enqueueErrorResponse.
• tests/payload_test.ts: tests payload parsing, depth caps, number precision, and streaming size limits directly against streams and byte arrays without HTTP scaffolding.
• scripts/messcript.mjs: adds payload to production quality gate scope.
• mutation/mutasaurus_ci.ts: adds tests/payload_test.ts to testFiles.