Pin mixed revisions between stored window boundaries

[zzylol]

Stacked on #811.

Why

Remote-write revision pinning required a stored window ending exactly at the query evaluation timestamp. Mixed queries between window boundaries fell back before bounded-lag execution could select the latest complete window.

What

Pin one compatible revision with complete stored windows within each mixed binding's configured lag bound (default one slide). Retain only admitted windows; stored-only queries still require exact interval coverage.

Before this PR

A mixed query evaluated three seconds after a window boundary fell back despite a complete stored window within its lag bound.

After this PR

The query fetches raw input at its requested timestamp, combines it with the admitted stored window, and reports the three-second lag. Incomplete newer windows do not shadow complete older windows.

Validation

• The new off-grid process regression failed before the fix and passes after it.
• Formatting, workspace clippy, workspace library tests, control-plane tests, and all 30 compatibility process tests pass.
• Independent agent review checked revision consistency, group completeness, lag bounds, and preservation of exact stored-only reads; no blockers remained.

🤖 Generated with Claude Code

Final stack validation at #809 / Planner a049a3b: formatting, clippy, workspace library tests, all control-plane tests, and all 30 process tests pass (122.91s).