fix(runtime): decode the shebang probe leniently so a multi-byte char at byte 256 can't hide the interpreter

[navbytes]

Summary

A plugin whose header comments contain an em-dash could fail vee lint and
vee render with a spurious non-zero exit, while running the same file directly
(python3 plugin.py) worked and printed valid output. The failure was
positional, not "em-dash present" — plugins with em-dashes elsewhere were fine,
which made it look content-sensitive and led to plugins being rewritten with
ASCII hyphens as a workaround.

The cause is in the shebang probe. PluginExecutor.shebang(of:) reads the first
256 bytes of the file and decoded them with a strict
String(data:encoding:.utf8). An em-dash is three bytes (E2 80 94); when one
straddled the 256-byte boundary, the truncated sequence made the entire decode
return nil. The shebang was then reported absent and launchCommand fell back
to /bin/bash <path> — so a Python plugin was handed to bash, which died with a
syntax error on the first print(...). That produced the non-zero exit in
vee lint, vee render, and the app itself (PluginCoordinator goes through
the same launchCommand).

The fix is to decode leniently with String(decoding:as:UTF8.self), the same
policy PluginSource already documents for reading plugin text: a truncated
tail becomes U+FFFD, which can only land past the first line, and the shebang
parses exactly as written. This was the only truncated read paired with a strict
decode in Sources/.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Plugin-format change (new line param / header tag / <vee.*> capability)
• Showcase example plugin (examples/) or SDK example (plugins/)
• Documentation only
• Refactor / internal change (no behavior change)
• Breaking change (fix or feature that changes existing behavior)

Testing done

• swift build
• swift test
• swift run vee (manual check in the menu bar)
• xcodegen generate + xcodebuild (if the app target is affected)
• npm test in plugins/ (if the TypeScript SDK is affected)

Regression test: ShebangLaunchTests.testHonorsShebangWhenMultibyteCharacterStraddlesReadBoundary
builds the straddling file byte-precisely (with a setup assertion that byte 255
is 0xE2, so the test can't silently stop covering the boundary). It fails
before the fix and passes after.

End-to-end, with a Python plugin whose header em-dash starts at byte 255:

# before
$ vee lint emdash.1m.py
Lint findings:
  error: plugin exited with code 2; only the output it managed to print was linted
$ vee render emdash.1m.py
emdash.1m.py: line 3: syntax error near unexpected token `'{"vee":1,...}''

# after
$ vee lint emdash.1m.py
No lint findings.
$ vee render emdash.1m.py      # exit 0

swiftlint lint --strict also reports 0 violations across 286 files.

Checklist

• Tests added/updated for the change (TDD — a bug fix has a regression test).
• All tests pass locally.
• No new third-party dependencies in the app or the SDKs.
• Code matches the surrounding Swift style; public API has doc comments.
• If plugin output/format changed, golden fixtures were regenerated and
  committed — n/a, no format change.
• Documentation updated as needed — n/a, the fixed behavior is what the docs
  already describe.
• Commits have clear, imperative subjects.