Remove stale data-loader and dynamic-import corpus rewrites

Parent tracking: #202 and #66.

The reduced Bun corpus runner still carries fixture-specific rewrites for JSON5, YAML, TOML, text-loader, and import-query coverage. Strict direct-VM execution already passes the unchanged upstream files. Routed execution should use a shared syntax-driven static data-loader import lowering path instead of exact fixture substitutions, while native dynamic import should make the text-loader and import-query rewrites unnecessary.

Acceptance criteria:

• Remove the five fixture-specific rewrite functions and dispatch branches.
• Preserve the upstream files byte-for-byte, including the original 10,000-import workload.
• Support extension-selected JSON5/YAML/YML/TOML static default imports and explicit supported import attributes generically.
• Pass all five upstream files in Debug and ReleaseSafe, both strict direct-VM and routed modes.
• Preserve the same test and assertion counts as pinned Bun.

Completed in ee0b5ee.

Removed the five fixture-specific JSON5, YAML, TOML, text-loader, and import-query rewrite paths. Static default imports selected by .json5, .yaml/.yml, or .toml extensions, plus supported explicit import attributes, now use one syntax-driven loader bridge.

Validation on the final rebased tree:

• pinned Bun: 35 pass, 0 fail, 104 expect calls
• Home Debug strict: 35 pass, 0 fail, 104 expect calls
• Home Debug routed: 35 pass, 0 fail, 104 expect calls
• Home ReleaseSafe strict: 35 pass, 0 fail, 104 expect calls
• Home ReleaseSafe routed: 35 pass, 0 fail, 104 expect calls
• Debug and ReleaseSafe builds succeeded
• all five upstream test files are byte-identical to the pinned Bun checkout
• the original text-loader 10,000-import workload remains intact
• no fixture, assertion, deadline, or workload changes; no heap snapshots