Implement exact PicoHeaders FetchHeaders adapter

Parent: #377

Implement WebCore__FetchHeaders__createFromPicoHeaders_ against Bun revision 4982b91e3702094330f3be3883354c52b8c01323.

Pinned layout:

typedef struct { const unsigned char *ptr; size_t len; } ZigSliceString;
typedef struct { ZigSliceString name; ZigSliceString value; } PicoHTTPHeader;
typedef struct { const PicoHTTPHeader *ptr; size_t len; } PicoHTTPHeaders;

Acceptance:

• Copy all accepted name/value bytes into the native FetchHeaders record; never retain foreign spans.
• Match the pinned adapter behavior for empty rows, known/uncommon names, duplicates, and Set-Cookie rows.
• Reject null or structurally invalid spans safely without dereferencing them.
• Add an exact-layout compile-link-runtime fixture in Debug and ReleaseSafe, refresh inventories/docs/CI, and update parents.

Implemented and pushed in 91c91b3f (adapter + inventories) and 42bcd356 (fixtures + docs).

Pinned behavior now covered:

• exact nested {ptr,len} ABI sizes and offsets from Bun 4982b91e
• all foreign name/value bytes are copied before return
• known duplicates combine; Cookie uses ; ; Set-Cookie remains multi-row
• uncommon duplicates replace the value while retaining the first spelling
• raw parsed values are not WebIDL-trimmed; empty name/value rows are skipped
• null, misaligned, null nested spans, oversized spans, and address overflow return an empty handle before mutation

Evidence:

• FetchHeaders units: 3/3 Debug
• Bun exact boundary: 22/22 Debug and ReleaseSafe
• Home exact private fixture: 385/385 Debug and ReleaseSafe
• inventories: Home 398/471, Bun 390/461, zero unclassified