Fix both-mode poll scheduler starvation after delayed reads

[FtlC-ian]

Justin's Expert 2K-FA packet captures exposed a deterministic bug in the both polling scheduler. When the serial read loop wakes after an even multiple of halfInterval, the catch-up loop advances through an even number of missed slots and returns to the same phase. Repeated timing at that cadence can starve one poll type indefinitely.

Observed on hardware:

• 143 display polls and 0 status polls in one session
• 3 display polls and 84 status polls in another
• the status-starved and display-starved directions both occurred

Scope:

• Make both mode fair under delayed read-loop wakeups and missed deadlines.
• Emit at most one due poll per scheduler call; do not create an unbounded catch-up burst.
• Preserve the configured aggregate cadence and deterministic status/display alternation.
• Preserve off, status, and display behavior and dynamic mode changes.
• Add regression tests for repeated delayed wakeups at even and odd multiples of halfInterval.
• Document the corrected bounded/fair behavior if current docs describe scheduler semantics.

This issue is independent of the separate SPE-LAN-UNIT finding that 0x90 receives no reply. Do not weaken fan-policy protocol-native safety gates here. PR #34 remains blocked pending direct-USB versus LAN-unit evidence and a fresh hardware validation.

[FtlC-ian]

Branch: fix/issue-38-poll-scheduler-fairness
Commit: c3f4c00b962cb414dc912f8331edc7e9fbcff7bc
PR: #39
Status: ready for review

The both scheduler now skips missed deadlines without advancing the pending poll phase, so delayed serial reads cannot starve status or display and catch-up remains bounded to one poll per scheduler call. Added repeated even/odd delayed-wakeup regression coverage.

Verified on the public tree with targeted scheduler/runtime tests repeated 20 times, go test ./..., go test -race ./internal/runtime, go vet ./..., git diff --check, private/public stable patch-ID parity, and independent exact-SHA review. Fan-policy and Menu Debug protocol-native safety gates are unchanged.

[FtlC-ian]

Branch: fix/issue-38-poll-scheduler-fairness
Commit: c3f4c00b962cb414dc912f8331edc7e9fbcff7bc
PR: #39
Merge commit: 3b60dc6ab4b89fed9b0955b8152f79964c3f155b
Status: merged to main

The both scheduler now skips missed deadlines without advancing the pending poll phase, so delayed serial reads cannot starve status or display and catch-up remains bounded to one poll per scheduler call. Added repeated even/odd delayed-wakeup regression coverage.

Verified on the public tree with targeted scheduler/runtime tests repeated 20 times, go test ./..., go test -race ./internal/runtime, go vet ./..., git diff --check, private/public stable patch-ID parity, independent exact-SHA review, and successful Cursor Bugbot. Fan-policy and Menu Debug protocol-native safety gates are unchanged.