[internal review, not for upstream yet] docs: state the anchored additivity invariant

[ColinDKelley]

Internal-review draft

This is the first of two upstream-bound PRs that follow up on prometheus/prometheus#18679. It is filed against Invoca/prometheus:prom-18679/upstream-snapshot (a pinned mirror of prometheus/prometheus@e793b2671) so the diff shows only the proposed upstream change. Do not push this branch upstream until we've reviewed.

The companion proposal-text amendment is here: Invoca/proposals#1.

What this changes

docs/feature_flags.md, inside the ### anchored subsection: appends a short callout that states the additive property of anchored-mode increase / rate / delta. The callout points at upstream issue prometheus#18679 for the precise statement and proposed tests.

Rendered preview: docs/feature_flags.md on this branch

Why

• The proposal already lists "Improved composability across range boundaries" as a use case (proposal 0052, line 222), but neither the proposal nor the user-facing docs say what exactly "composable" means.
• We've run yincrease / yrate / ydelta in production for ~3 years and the additive property is the reason it stays predictable and explainable under irregular scrapes, with no partition-boundary surprises. Putting it in writing here in feature-flag doc makes the guarantee discoverable.
• Sets up the proposal-amendment PR to anchor on a one-line invariant rather than re-deriving it.

Review checklist

• Math reads cleanly in GitHub's Markdown render (Unicode subscripts on T₀ T₁ T₂, r₁ r₂)
• Wording matches the issue body at PROM-52: formalize composability with the 'additive' property; add tests to prove the property prometheus/prometheus#18679
• No accidental edits outside the anchored block
• Tone fits the surrounding feature-flag entries (we keep it short and link out rather than restating the invariant here)

Once approved

• Retarget base to prometheus/prometheus:main
• Convert from draft to ready-for-review
• Cross-reference from the upstream issue thread

[ColinDKelley]

Internal review complete. Squashed + DCO-signed and filed upstream as draft: prometheus#18690. Closing this internal PR.