Split oversized dynamic-expression commits by feature ownership

[stevenvar]

Context

The clean dynamic-expression history is ordered coherently, but two commits still cover multiple implementation layers and are difficult to review independently. This is a history-maintenance task; it does not describe a known behavior bug.

Proposed split

15f1c18a0c1 Propagate dynamic expressions through tf2xla kernels

Separate:

1. Shared tf2xla expression plumbing and helper APIs.
2. Individual kernel expression-propagation conversions.

The plumbing should compile before the kernel conversions are introduced, and tests should remain with the behavior they cover.

3d1a28c56e4 Compile and execute clusters with dynamic expressions

Separate:

1. Runtime expression substitution and solving at cluster boundaries.
2. Batch bucketing, padded compilation arguments, and executable-cache policy.

The split must preserve dependency order and leave every intermediate commit buildable.

Constraints

• Do not change the final source behavior.
• Do not introduce formatting or include cleanup unrelated to the moved code.
• Keep tests in the earliest commit where their tested behavior exists.
• Re-run the focused dynamic-expression tests at each new boundary before replacing the published history.