experimental/air: content-address the plain_tar upload - #6578

## Summary

Makes the `plain_tar` upload content-addressed so an unchanged working
tree skips packaging **and** upload.

Today `git_archive` names its tarball by `(commit, include_paths)` and,
if that object is already uploaded, reuses it and moves no bytes.
`plain_tar` (dirty working tree / no ref — the normal
iterate-and-resubmit loop) used a **timestamped** name, so it
re-packaged and re-uploaded the full tarball on *every* submission, even
when nothing changed.

This change:

- **`snapshotFiles`** records each path's size, mtime, mode/type, and
symlink target. Size, mtime, and mode/type come from the existing
`Lstat`; only symlinks require an additional `Readlink`.
- **`computePlainTarKey`** hashes the sorted
`path+size+mtime+mode+symlink_target` set into a versioned working-tree
fingerprint.
- **`snapshotTarName`** names `plain_tar` `<dir>_<fingerprint>.tar.gz`
(was `<dir>_<timestamp>.tar.gz`) and returns the file listing so
packaging reuses it — the tree is walked once, not twice.
- **`uploadSnapshotViaDABs`** runs the existing `snapshotExists` skip
for **both** modes: if the object already exists, it reuses the remote
path and uploads nothing.

## Validation

- `go test ./experimental/air/cmd` passes. Tests cover remote
deduplication and verify that path, size, mtime, mode/type, and
symlink-target changes independently alter the cache key.
- `./task lint-q` passes with zero issues.
- **End-to-end on df1.** Submitting an unchanged universe tree twice:
the second run logged `snapshot upload skipped; reusing
…<fingerprint>.tar.gz`, returned the identical remote
`code_source_path`, and skipped packaging + upload. Both runs launched
(1×A10):

| code source | first submit (package + upload) | second submit
(unchanged → skipped) |
  |---|---|---|
  | `research` (~27 MB) | 5.7 s | 4.9 s |
  | `research` + `js` + `spark` (~97 MB) | 37.6 s | 4.8 s |

These measurements preceded the parallel gzip improvement in #6571, so
the first-submit numbers are historical; the unchanged-tree skip remains
the behavior demonstrated here.

## Notes / trade-offs

- Regular-file content detection is size+mtime, not content — the same
trade-off DABs file sync makes — so an edit preserving both is not
detected. Mode/type and symlink-target changes are detected.
- Remote objects accumulate (one per distinct tree state), same as
`git_archive`; no eviction is added here.

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This pull request and its description were written by Isaac.

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Co-authored-by: Isaac <no-reply@databricks.com>