add asset_key_pattern to the s3 builders

[jaemk]

• Add asset_key_pattern(..) to the s3 Update/ReleaseList builders: a custom regex for deriving (name, version) from object keys, replacing the built-in matcher whose version group only captures a major.minor.patch triple. A pre-release key like mybin-0.1.2-beta-x86_64-... can now parse as 0.1.2-beta instead of 0.1.2.
• Require name and version named capture groups; compile and validate the pattern at build(), surfacing failures as the new Error::InvalidAssetKeyPattern (s3 feature, source chained).
• Skip keys whose captured version does not parse as semver (same treatment as the forge backends' non-semver-tag skip). Unset keeps the default matcher's behavior unchanged, pinned by a regression test.
• Add parse, builder-validation, and loopback-stub end-to-end tests; update specs/ref-s3-backend.md, specs/ref-errors.md, and the CHANGELOG.

Closes #61