Bun parity: lower file-attribute imports for compressed fetch

Port js/web/fetch/fetch.brotli.test.ts from the copied Bun corpus.

Required behavior:

• lower default static imports using with { type: file } to stable corpus-relative asset paths
• load the real Brotli and Gzip fixture bytes through Bun.file
• auto-decompress Brotli and Gzip fetch response bodies to identical text
• preserve Content-Encoding metadata when decompress is false
• retain structured file-read and response-decompression error causes and composed stacks
• add a permanent focused regression and advance corpus position 4148 with zero failures or unsupported tests

Tracked under #66.

Completed in 0fb9c20 and pushed directly to main.

Implemented:

• reusable lowering for default static imports with with { type: file }
• stable corpus-relative asset path resolution with query stripping
• real Brotli and Gzip fixture loading through Bun.file
• automatic Brotli/Gzip response decoding with identical decoded text
• Content-Encoding preservation for decompress: false
• permanent focused regression retaining structured file-read and fetch.response.decompress failures

Validation:

• indexed Bun corpus position 4148: 1 passed, 0 failed, 0 todo, 0 unsupported
• focused compressed fetch file-import regression: passed
• zig build --cache-dir /private/tmp/home-zig-abi-cache: passed