seal: pace SDR Do entry with cancellable reservations

[hyunmoon]

Draft — depends on #1533. Files changed includes the foundation; incremental review shows only this PR's changes.

Summary

Add optional process-local minimum SDR start spacing and stable phase without blocking the scheduler. Speculative acceptance does not consume spacing. Pre-entry failures release only their reservation; committed Do entry consumes the interval even if execution later fails. Snapshot diagnostics under the mutex and log after unlocking. Do not sleep in scheduling or hold resources while waiting for pacing.

Zero/default is unpaced. Restart loses in-memory spacing, and phase hashes can collide: this is not a cluster-wide rate limiter. No storage-capacity, UI or native-backend policy is included.

Validation and limits

Normal/race/configuration tests and deterministic synthetic lifecycle scenarios across multiple capacities and instances cover reservations, hold, completion and refill. The probe uses substitute SQL/storage/native bodies; it does not establish native SDR performance. Actual completion-SQL validation is described in the parent PR.

Review scope

Review boundary	Commits	Diffstat
Own changes, after foundation	1	14 files changed, 1366 insertions(+), 2 deletions(-)
Cumulative against upstream main	4	58 files changed, 7155 insertions(+), 418 deletions(-)

Series overview. No dependency on the other children. Ready after the foundation is accepted, its duplicated diff is removed and applicable checks pass.

Own commits and comparison base

Own commits:

• 48bce9af395204a3820cb42a4817468ae8c5cdcf — seal: pace SDR Do entries with reservations and nonblocking diagnostics

Cumulative counts use upstream 86e95967cd602dbf8db5ea538abe986c3968e2e3; the incremental base is 0c1f7cbd4dc9797e9773f1edd165af6b72d3fe27.