fix(wasi): component file read returns bytes in a guest-owned buffer (SR-38, #405)

[avrabe]

What

Second layer of #405. After SR-37 (#406) a preview2 component could open+stat a file under --wasi-fs but reading it crashed with [Runtime][E07DA] Function not found in exports.

Root cause (measured, trace-driven)

wasi:io/streams input-stream.blocking-read wrote the returned list<u8> to a fixed raw address (0x100000 + handle*0x10000) the guest never allocated. wasi-libc copies then frees the returned list — freeing an unallocated pointer corrupted the guest allocator → bad indirect call → E07DA. It also re-read the whole file from offset 0 every call with no EOF, so the read loop never terminated.

Fix

The component WASI path is dispatch_canon_lowered, which auto-lowers a handler's returned core values to the retptr via lower_results_to_retptr using the Result(ListU8, stream-error) signature. So:

• blocking-read/read now return core values [I32(0), buf_ptr, n] (ok) / [I32(1), I32(1)] (stream-error::closed) — no manual retptr write.
• The bytes live in a fresh cabi_realloc'd buffer per read (allocate_wasi_read_buffer, sized to the requested len, capped 1 MiB), set via new HostImportHandler::set_read_buffer_allocation. On-demand allocation added in dispatch_canon_lowered (component path) and call_wasi_function (core-module path).
• A per-stream read offset advances by bytes returned and yields closed at EOF.

Verification

• Read-only preview2 component now prints READ:CANARY and completes cleanly, matching wasmtime run --dir .::..
• 2 unit oracles (// rivet: verifies SR-38): read returns bytes in the owned buffer + EOFs; fails loud without a buffer.
• kiln-runtime lib suite: 95/95 pass (engine changes clean).

Scope

#405 stays open — directory enumeration (read-directory / read-directory-entry) is still unimplemented, filed as SR-39. v0.4.1 cuts when that lands and closes #405 end-to-end.

Trace: SR-38

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Summary

Metric	Base Branch	This PR	Change
Errors	0	0	0
Warnings	5	5	0

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