Port complete gRPC Metadata semantics

Scope

Port Bun corpus position 3984, js/third_party/grpc-js/test-metadata.test.ts (29 active tests).

Required contracts

• validate and normalize metadata keys
• require strings for ordinary keys and Buffers for -bin keys
• reject non-ASCII ordinary values
• implement set/add/remove/get/getMap
• clone metadata without sharing binary buffers
• merge without mutating the source
• convert to and from HTTP/2 headers, including base64 binary values
• preserve validation/conversion causes with operation, key, and source-module stack context

Verification

• all 29 focused upstream tests pass without unsupported cases
• permanent diagnostics cover validation context and binary clone isolation
• existing gRPC, SQL adapter, and third-party gates remain green

Implemented and pushed in cb3dc9144.

Completed contracts:

• normalized and validated metadata keys
• enforced string values for ordinary keys and Buffers for -bin keys
• rejected non-ASCII ordinary values
• implemented set/add/remove/get/getMap and metadata options
• deep-cloned binary buffers
• merged metadata without mutating the source
• converted to/from HTTP/2 headers with base64 binary values
• added the missing Node assert.notDeepStrictEqual primitive used to verify clone isolation
• validation/conversion errors retain causes and add metadata operation, key, and source-module stack context

Verification:

• focused Bun corpus: 29 passed, 0 failed, 0 todo, 0 unsupported
• permanent gRPC diagnostics: 11 passed
• SQL adapter contracts: passed
• permanent third-party JWT/utility mini-suite: passed
• zig fmt --check and git diff --check: passed

Cumulative through position 3984: 326 files, 2,693 passed, 209 todo, 0 failed, 0 unsupported, 171 allowed-empty.