Port Bun S3 ListObjectsV2 request and XML semantics

Tracks the focused compatibility tranche for js/bun/s3/s3-list-objects.test.ts under #66 and #81.

Scope:

• canonical ListObjectsV2 option ordering and percent-encoding
• static versus instance endpoint path behavior
• signed request headers and request-option overrides
• typed invalid-argument, missing-credential, and S3 response errors
• parse scalar fields, contents, owners, and common prefixes from S3 XML
• keep malformed and very large response parsing bounded
• activate the exact copied fixture and add a permanent Home regression gate

The copied fixture has 35 deterministic local tests plus a credential-gated live-R2 suite.

Completed in dd63b2218.

Implemented canonical ListObjectsV2 query ordering/encoding, static and instance endpoint paths, signed request overrides, typed argument/credential/S3 response errors, and bounded forward XML parsing for scalars, contents, owners, and common prefixes.

Verification:

• exact copied js/bun/s3/s3-list-objects.test.ts: 35 passed, 0 failed, 0 unsupported
• the fixture’s sole TODO is its own secret-gated live-R2 suite
• 40,000-object response and 5 MB malformed XML performance regression pass
• ./pantry/.bin/zig build test -Dfilter=home_test --summary failures: passed

This advances #66 and #81.