fix: preserve Blob values with files preserve

[samlaycock]

Summary

Fixes #66.

This updates plain-object encoding so Blob values are handled consistently with File values:

• preserves plain object Blob values when { files: "preserve" } is passed
• keeps the default behavior explicit by throwing for plain object File and Blob values unless preservation is enabled
• widens the preserved encode entry type to [string, string | File | Blob][]
• updates README API/docs to describe the Blob support
• adds a patch changeset for the behavior fix

Verification

• bun fmt
• bun lint:fix
• bun test
• bun typecheck

[greptile-apps]

Greptile Summary

This PR fixes issue #66 by extending the files: "preserve" option to handle plain-object Blob values alongside File values. It also closes the previously reported type mismatch by widening decode's parameter from Iterable<[string, FormDataEntryValue]> to Iterable<DecodableEntry> (where DecodableEntry = [string, string | File | Blob]).

• isFileValue in encode.ts now detects Blob in addition to File, with the same environment guard (typeof Blob !== \"undefined\"), and the walk-time error message is updated accordingly.
• DecodableEntry / DecodableEntryValue are introduced in decode.ts and exported from both core.ts and index.ts, making the widened decode signature part of the public API.
• Tests cover the new encode-preserve, encode-throw, and reserved-key error cases for Blob; a compile-time assertion in types.test.ts confirms the encode → decode type round-trip now type-checks.

Confidence Score: 5/5

Safe to merge - the change is additive and well-scoped with no impact on existing encode/decode behavior for non-Blob inputs.

The Blob preservation path follows the exact same pattern as the pre-existing File preservation path, the type widening is correct in both directions, and the only gap is a minor decode error-message inconsistency that does not affect runtime behavior.

No files require special attention beyond the minor error-message inconsistency in src/decode.ts.

Important Files Changed

Filename	Overview
src/encode.ts	Widens PreservedFileEntry to include Blob, updates isFileValue to detect Blob, updates error message, and broadens encodeStringEntries parameter type - all changes are correct and consistent.
src/decode.ts	Adds DecodableEntry/DecodableEntryValue types and widens the decode parameter to accept Blob entries; the thrown error message still reads 'File entries are not supported' instead of 'File and Blob entries'.
test/roundtrip.test.ts	Adds encode-side tests for Blob preserve and throw cases.
test/types.test.ts	Adds a compile-time assertion verifying encode output with Blob entries is assignable to the decode parameter type.
src/core.ts	Re-exports the new DecodableEntry and DecodableEntryValue types - straightforward additive change.
src/index.ts	Adds DecodableEntry and DecodableEntryValue to the public API surface.
README.md	Updates docs to mention Blob alongside File throughout and corrects the API signature table.

_{Reviews (2): Last reviewed commit: "fix: accept blob entries in decode types" | Re-trigger Greptile}