OCapN: improved error messages and testing

[kumavis]

some small improvements:

• strict TextDecoders
• error chaining in codecs
• improved test coverage

related to @endo/pass-style support of "selector" (#2774)

[Copilot]

Pull Request Overview

This PR improves error messages and testing support across the codebase by adding error wrapping (with nested causes) and stricter decorator-based validations, as well as expanding test coverage for various codecs and passable values. Key changes include:

• Wrapping codec read/write methods with detailed error messages.
• Updating test expectations to use a new throws utility and a less-strict TextDecoder for fuzz tests.
• Renaming and prefixing codec identifiers for clarity and consistency.

Reviewed Changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/ocapn/test/syrup/*	Updated tests to use a non-fatal TextDecoder for fuzz descriptor and standardized error expectations via throws utility
packages/ocapn/src/syrup/*	Refactored codec implementations with error wrapping and added codec name parameters
packages/ocapn/src/codecs/*	Improved error chaining, renamed codecs with consistent prefixes, and updated union codec implementations
packages/ocapn/src/syrup/buffer-reader.js	Fixed a formatting bug in an error message
packages/ocapn/test/_table.js & _syrup_util.js	Adjusted test table entries and moved utility functions for consistency

Comments suppressed due to low confidence (2)

packages/ocapn/src/syrup/buffer-reader.js:110

• The error message now includes a closing parenthesis, which fixes the previous formatting issue. Verify that all similar error messages follow this corrected punctuation for consistency.

        `End of data reached (data length = ${state.length}, asked index ${index})`

packages/ocapn/test/syrup/_table.js:13

• There is a duplicate table entry with syrup '1-' and value -1n. Confirm whether this duplication is intentional or if the redundant entry should be removed.

    { syrup: '1-', value: -1n },

[kumavis]

@erights in order to avoid blocking this PR on @endo/pass-style Selector support, i have added a file to provide the minimal functionality needed from @endo/pass-style for this early implementation of OCapN (currently focused on message parsing). After Selector support is added to @endo/pass-style this file will be easily replaced with the methods of the same name provided by @endo/pass-style