docs(rfc-0033): record Phase 3 as implemented

[ragnorc]

Records in RFC 0033 that Phase 3 is done: the engine writes (#908), PUT and DELETE /blob (#922), and blob put / blob clear (#928).

• Header and §13: Phase 3 is implemented for v0.13.1, with the PRs that delivered each step.
• §10.1: "The ceilings are not a setting" is no longer true since write_max_bytes. It now says a setting can lower the ceilings but not raise them, and 32 MiB is its maximum.
• Phase 4: the remaining benchmark needs a harness that can drive Blob reads and export. The active benchmark catalog runs GQ operations only, and neither is one. Until then, export.rs::export_reads_each_batch_of_managed_blobs_in_one_read_per_column pins the read count.
• Decision log: a 2026-10-11 entry quotes the superseded sentences.

Docs only; check-docs.py and typos pass.