refactor(poll_descriptor): split debug_log_throttle submodule (#4404 inc 1)

[psaab]

Addresses #4404 (increment 1: extract/split debug_log_throttle; further increments tracked).

What

Pure code-motion split of the cold-path debug-log throttle unit out of
userspace-dp/src/afxdp/poll_descriptor/mod.rs into a new sibling submodule
poll_descriptor/debug_log_throttle.rs.

Moved verbatim:

• the two code-quality: leftover test-env 'is_trust_flow' hardcode (ifindex==5 || zone=='lan' || 10.x src) gates debug logging in the session-miss cold path — defeats the dbg rate-caps + floods logs on any real 10.x LAN (fable-163 F27) #4120 interval caps SESSION_MISS_DEBUG_LOG_CAP / POLICY_DENY_DEBUG_LOG_CAP (kept module-private),
• the two pure u64 -> bool predicates session_miss_debug_log_allowed / policy_deny_debug_log_allowed (now pub(super) so the poll_binding_process_descriptor call sites still reach them),
• the debug_log_throttle_tests pinned-contract test module (code-quality: leftover test-env 'is_trust_flow' hardcode (ifindex==5 || zone=='lan' || 10.x src) gates debug logging in the session-miss cold path — defeats the dbg rate-caps + floods logs on any real 10.x LAN (fable-163 F27) #4120 RED-on-revert guard).

mod.rs gains a mod debug_log_throttle; declaration + a use import.

Why this unit / why pure code-motion

This unit has zero cross-references back into mod.rs — the predicates depend on no
dataplane type — so it is the cleanest available boundary, and it directly matches the
issue's stated fix of outlining the cold-path debug logging. Behavior, the public
visibility surface, and all three test cases are identical; the bodies move verbatim.

The poll_binding_process_descriptor god-function itself (session-hit / session-miss /
flowless arm fusion) is not touched — per the issue it needs /triple-review because
it carries the single-recycle, CoS guarantee-guard, and packet-range/verifier-sensitive
invariants. Those further increments remain tracked on #4404.

Validation

Full cargo test (CARGO_TARGET_DIR=/tmp/cargo-4404) passes across every binary:

• main lib: 3739 passed / 0 failed (the 3 moved tests now run as afxdp::poll_descriptor::debug_log_throttle::debug_log_throttle_tests::*)
• other binaries: 60 + 8 + 22 + 1 passed, 0 failed

New file is rustfmt-clean at edition 2024; mod.rs changed lines match surrounding style.
Module doc (mod.rs header) + _Log.md updated.

🤖 Generated with Claude Code

https://claude.ai/code/session_015oARShYtiJJ2H4UB4nXGqi

[Copilot]

Pull request overview

Refactors the afxdp::poll_descriptor module by extracting the cold-path debug-log throttle caps/predicates (and their pinned-contract tests) into a dedicated sibling submodule, keeping poll_binding_process_descriptor logic unchanged while reducing local clutter and isolating a dependency-free unit.

Changes:

• Added poll_descriptor::debug_log_throttle submodule and re-exported the two throttle predicates to the parent module via pub(super).
• Removed the throttle caps/predicates and their tests from poll_descriptor/mod.rs, wiring call sites through a use import.
• Logged the refactor increment in _Log.md per repository practice.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
userspace-dp/src/afxdp/poll_descriptor/mod.rs	Declares/imports the new debug_log_throttle module and removes the inlined throttle unit/tests (pure code motion).
userspace-dp/src/afxdp/poll_descriptor/debug_log_throttle.rs	New module containing the throttle caps, predicates, and the pinned-contract tests moved verbatim.
_Log.md	Records the #4404 increment-1 refactor action and validation summary.