Implement FetchHeaders core lifecycle and buffer ABI

[chrisbbreuer]

Parent: #375

Implement the 21 FetchHeaders exports that do not dereference UWS/H3/Pico request or response objects: branding/casts, JS conversion, native lifetime, create/clone paths, StringPointer creation and copying, validated mutations and queries, counts, and fast HTTPHeaderName operations.

Acceptance:

• One branded ordered Headers representation backs JS and native handles.
• Standalone createEmpty/createValueNotJS handles have explicit ownership and deref behavior.
• createFromJS, createValue, toJS, clone, and cloneThis preserve VM ownership and exact exception semantics.
• append/set/get/has/remove and fast operations match Fetch normalization, combination, Set-Cookie, and enum mapping.
• count/copyTo report exact iterator rows and UTF-8 buffer sizes with checked u32 bounds.
• Add Debug and ReleaseSafe Home/Bun compile-link-runtime evidence, refresh inventories/docs/CI, and push as one focused main commit.

[chrisbbreuer]

Implemented and pushed in 975386e2 (core) and b4a9f717 (fixtures, CI, and docs).

Evidence:

• one branded, independently ref-counted native record backs JS Headers and the C ABI
• all 21 scoped exports pass the Bun runtime matrix (21/21)
• Home private ABI fixture passes 384/384 in Debug and ReleaseSafe when run in isolation
• focused Headers tests pass 4/4; FetchHeaders unit tests pass 2/2
• inventories now classify Home 397/471 and Bun 389/461
• the parallel-build shared-memfd flake is isolated and tracked separately in ABI fixture: make shared-memfd test reliable under parallel build load #379

The four foreign-object adapters remain intentionally out of this core slice under #377.