Add shared EnsureGeometry to storage/internal

[roger2hk]

This PR only adds the shared API. Nothing calls it yet:

• PartialResourceIO: the tile and entry bundle I/O, provided by each storage implementation.
• EnsureGeometry: ported from storage/posix (ensureGeometry, ensurePartialBundle, ensurePartialTile, maxLevel).
• MarshalTlogEntryBundle: ported from storage/posix (marshalTlogEntryBundle).

Differences from the existing POSIX implementation:

• I/O is injected via PartialResourceIO.
• The existence check uses an exact read instead of stat. These reads must not fall back to the full resource, as LogReader.ReadTile/ReadEntryBundle do. Otherwise, whenever the tree's tile is already full, a missing partial is never created.
• Source resources are length-checked before truncation, so short or corrupt data returns an error instead of panic.
• Errors are wrapped with %w.

The next few PRs will wire POSIX and GCP to this new API.