micro_scheduler: targeted get_next_batch calls must not pop the TP-follow FIFO

[vasilevklart]

Problem

_try_schedule_tp_follow pops the leader's ScheduleTPNode from the FIFO before returning the batch. A TP-follow batch is a mandate: rank 0 has already committed to it and blocks on the collective inside the forward until every follower joins (per the existing note in get_next_batch), and a popped message has no re-queue path — followers cannot initiate scheduling for parallel nodes.

The targeted call path (target_node_name / target_graph_walk, used only by the speculation fresh-rid merge in _try_speculate_next) is a discretionary consumer: it may reject rids from the batch it is handed (the "in-flight rid wins" branch). Serving it from the TP FIFO can therefore strand a popped message — the follower never runs the batch, and the TP group hangs at the next collective. This is the KNOWN GAP (latent) note in _try_speculate_next: unreachable today only because spec targets exclude parallel nodes, reachable the moment speculative scheduling is enabled for lockstep-parallel nodes (the "disable … for now" in _can_speculate).

Change

Refuse the combination at the source: targeted calls never touch the TP-follow FIFO; the message stays queued for the unconditional scheduling path. This replaces the per-target filters, which only made the overlap partially safe. exclude_target keeps its filter — its caller (the yield-away path) submits what it receives unconditionally.

No behavior change in any current serving path: with speculation disabled for parallel nodes, no targeted call can be handed a TP-follow batch today. This hardens the invariant ahead of that work.

Tests

test/modular/test_tp_follow_targeted_gate.py:

• targeted calls — including an exactly-matching target — return nothing and leave the FIFO intact
• untargeted calls still serve and drain the FIFO
• exclude_target skips the head but keeps it queued
• FIFO order survives refused targeted calls

The two gate tests fail without the fix; the two behavior-preservation tests pass with and without it.