Expose a pre-execution seam in @stx.session for pre-flight compute checks (scitex-clew P3)

[ywatanabe1989]

Context

scitex-clew has shipped a pre-flight compute estimate (scitex-compute-preflight-check):

• Phase 1 (merged, scitex-clew#64): clew estimate <script> predicts runtime p50/p90 + success rate + #outputs from run history.
• Phase 2 (merged, scitex-clew#65): adds size_bytes + predicted output volume and a cached-intermediate reuse hint.

The end goal (operator-requested) is a pre-run prompt: before a heavy compute/figure script executes, warn "this is a lot (~Xm, ~N files, ~Y MB) — are you sure? consider subset / sbatch / GPU / reuse-cached." This mirrors scitex-writer's pre-COMPILE pre-check framework, on the COMPUTE side.

The ask

clew can compute the estimate today (scitex_clew.estimate(script_path)), and it already owns the session lifecycle hooks (on_session_start / on_session_close, wired via clew's observers). What's missing is a pre-execution seam in scitex-dev's @stx.session decorator (scitex_dev.decorators): a hook point that fires AFTER the session is identified but BEFORE the decorated body runs, where a registered callback can:

1. receive the script path / session context,
2. optionally emit a warning + return a decision, and
3. when interactive (tty) and over a threshold, prompt the user; when non-interactive/CI, no-op (must never block batch jobs).

Proposed shape (open to your design)

• A pre_run / before_execute callback registry on @stx.session (analogous to the existing start/close observer mechanism clew already plugs into), OR
• A documented pre_execute_hooks list the decorator invokes before the body.

clew would register a callback that calls scitex_clew.estimate(...) and prompts. No import of clew from scitex-dev is required — clew registers into scitex-dev's hook, keeping the coupling one-way and optional (removing clew degrades to "no pre-flight," never breaks @stx.session).

Constraints

• Zero added cold-start cost when no pre-run hook is registered.
• Non-interactive / CI runs must short-circuit (no prompt, no block).
• One-way dependency only (scitex-dev must not import scitex-clew).

Tracking

scitex-clew side is carded as clew-preflight-p3 (blocked on this). Happy to implement the clew-side callback once the seam exists, and to pair on the hook signature.