stem: do not track credits for unreliable outs

[mmcgee-jump]

No description provided.

[Copilot]

Pull request overview

This PR updates fd_stem credit accounting so that output links without reliable consumers do not participate in flow-control credit tracking, and adjusts the gossip tile’s STEM_BURST sizing to reflect per-output worst cases.

Changes:

• Refines STEM_BURST in the gossip tile to bound per-output publishes between credit checks (reliable links only).
• Adds an out_reliable mask to fd_stem_context_t and uses it to skip credit decrement/min tracking for unreliable outputs.
• Extends fd_stem scratch layout/allocation to include the out_reliable array and marks outputs as reliable based on consumer wiring.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/discof/gossip/fd_gossip_tile.c	Reworks STEM_BURST definition/documentation to reflect per-link worst-case bursts for reliable outputs.
src/disco/stem/fd_stem.h	Adds out_reliable to the stem context and gates credit decrement/min tracking on it.
src/disco/stem/fd_stem.c	Allocates/initializes out_reliable in scratch and sets it based on consumer wiring; adjusts cr_max initialization.

[Copilot]

Setting cr_max to 128 when cons_cnt==0 makes stem treat the tile as permanently/backward-pressure limited (min_cr_avail will never reach burst if burst>128) and can also trigger the "insufficient depth for STEM_BURST" fatal error even though there are no reliable consumers. For the no-consumer case, cr_max/min_cr_avail should be set so that the run loop never backpressures (e.g., keep the previous ULONG_MAX behavior or explicitly bypass burst/credit checks when cons_cnt==0).

Suggested change

	ulong cr_max = fd_ulong_if( !out_cnt || !cons_cnt, 128UL, ULONG_MAX );
	ulong cr_max = (!cons_cnt)
	? ULONG_MAX
	: fd_ulong_if( !out_cnt, 128UL, ULONG_MAX );

[github-actions]

Performance Measurements ⏳

Suite	Baseline	New	Change
backtest mainnet-368528500-perf per slot	0.071413 s	0.071719 s	0.428% ✅
backtest mainnet-368528500-perf snapshot load	3.135 s	2.751 s	-12.249% ✅
backtest mainnet-368528500-perf total elapsed	71.413282 s	71.718913 s	0.428% ✅
firedancer mem usage with mainnet.toml	964.37 GiB	964.37 GiB	0.000% ✅

[github-actions]

Performance Measurements ⏳

Suite	Baseline	New	Change
backtest mainnet-368528500-perf per slot	0.074931 s	0.075573 s	0.857% ✅
backtest mainnet-368528500-perf snapshot load	4.308 s	3.087 s	-28.343% ✅
backtest mainnet-368528500-perf total elapsed	74.930722 s	75.57284 s	0.857% ✅
firedancer mem usage with mainnet.toml	964.37 GiB	964.37 GiB	0.000% ✅

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.