docs(plans): reverse the ruling against observability.md rows

Reverses this plan's ruling that no observability.md row is added for the inline-build metric families, so the plan and !2213 (merged) stop contradicting each other after merge.

!2213 (merged) adds five rows: both kill counters, the arm-failure counter, and both document-size histograms. The plan concluded the opposite, and a step MR may not edit the plan file, so the reversal lands here instead.

The reversal gives each group its own reason, because no single reason covers all five. The two kill counters name a 200 that was already committed when its body was cut short: the response cannot carry that failure, the client is never told, and the service-wide table is the only entry point an operator has to it. The arm-failure counter reports no truncation of its own, it reports the blind spot it opens in those two. The two document-size histograms observe a size on every fill-arm delivery that carries one, so they are almost entirely a record of deliveries that finished; they earn a row because ## Open Questions leaves them the only production signal of the five that does not turn on an unanswered question, and because a filling histogram beside a zero kill counter is what separates an absent defect from an unreachable code path.

Three companion corrections the reversal makes necessary:

• docs/dev/observability.md goes into step 1's Files list, which is where a step MR reads what it owns, with the two existing rows step 1 falsifies named alongside the five it adds. It goes into step 2's list too, because step 2 re-points what kill_outcome's budget_exhausted_before_body means on the stream arm, which is the correction step 2 already makes to metrics.md.
• The kill counters carry a kill_outcome label over a closed two-value set. The plan said they mirror the collapsed pair's shape, and that pair is a plain counter, so the Files entry and the Naming Conventions table described unlabeled counters while the reversal cited the label. Both now record the label and its two values.
• ### Open MRs touching the same files was written when its three MRs were open and all three have merged, so it now says so with dates and reads correctly either way. It also records that eight other MRs were open against observability.md on 2026-09-02, four of them editing the block step 1's rows land in. None touches an inline-build family, so the collision is textual and no ordering follows from it.

The plan's separate decision that backfilling the four existing families is its own docs change stays standing. The asymmetry that leaves is named in the amended text as the cost this reversal accepts.

Raised in review on !2213 (merged): !2213 (comment 3774434981)

Merge order: this merges first. !2213 (merged) waits for it.