OCaml rewatch performance and immutable interfaces

[fhammerschmidt]

Summary

This PR consolidates the OCaml Rewatch experiment into one reviewable stack, with one commit for each previously opened PR and the follow-up implementation work on top.

The changes improve incremental build performance and make the project the compiler's lifetime boundary. Rewatch now reuses compiler sessions, signatures, and runtime CMIs where possible, tracks build freshness and compiler configuration explicitly, and supports an immutable representation of compiled interfaces behind REWATCH_FROZEN_VALUES=1.

The latest follow-up work completes the build lifecycle changes and keeps compatibility file copies explicit. The normal build path avoids legacy source and generated-output copies; compatibility fixtures can opt in with REWATCH_COMPAT_COPIES=1. Compiler metadata records this mode so changing it invalidates stale artifacts correctly.

Commit structure

1. Improve OCaml Rewatch performance and benchmarking (from Improve OCaml rewatch performance and benchmarking #8671)
2. Add exclusive compiler phase tracing (from Add exclusive compiler phase tracing for OCaml rewatch #8672)
3. Reuse compiler signatures and runtime CMIs (from Reuse compiler signatures and runtime CMIs in OCaml rewatch #8673)
4. Make the project the compiler lifetime boundary (from Make the project—not the module—the compiler’s lifetime boundary #8675)
5. Add an immutable representation of compiled interfaces (from Immutable representation of compiled interfaces #8676)
6. Continue the immutable-interface experiments with broader compiler-state and session coverage
7. Advance the Rewatch lifecycle and tooling performance
8. Gate compatibility copies behind an explicit mode

Validation

• make checkformat
• make test
• make test-rewatch
• OCaml Rewatch integration tests, including changed interfaces after failed implementations

Supersedes

#8671, #8672, #8673, #8675, and #8676 are represented in this single review. This PR is stacked directly on #8653 (rewatch-ocaml), which remains the separate “Rewrite Rewatch in OCaml” review.