server: context checkpoints ride with a slot save/restore — a restored slot is warm again

[joelteply]

Why

/slots/N?action=restore brought back KV + tokens but prompt.clear() dropped the context checkpoints. On a hybrid/recurrent cache the next request can reuse a prefix only by rolling back to a checkpoint at or before its divergence (even an identical prompt rolls back one token, [TAG_PROMPT_LOGITS]), so a restored slot always hit do_reset: "restore ok" then a full re-prefill. Measured on Ornith-1.5-35B-A3B, 2026-09-04: n_restored 11298 → cache_n 0; the second identical request 11291. This is what made Continuum's KV context-switch (page to disk on eviction, restore on return) cost a full prefill on every return.

Change (server-context.cpp only, +73)

• SLOT_SAVE writes <file>.ckpt beside the state file: magic/version/count, then per checkpoint n_tokens, pos_min, pos_max and the three state blobs (data_tgt, data_dft, data_spec).
• SLOT_RESTORE reads it back into slot.prompt.checkpoints. A missing or malformed sidecar leaves exactly the pre-change behaviour (valid restore, no checkpoints) with a warning — never an error.

Acceptance (Continuum side)

serving/cache-probe --roundtrip: prime → save → restore → same prompt must report cache_n ≈ n_restored on the FIRST request after the restore (today: 0).

🤖 Generated with Claude Code

https://claude.ai/code/session_01LoTjvf5j3Ez13g6k8mRkFo

[joelteply]

A/B on the M5 (Ornith-1.5-35B-A3B Q4_K_M, Metal, --cache-ram 0 so the disk restore is what is measured; 1321-token prompt; prime → save → clobber the slot with another prompt → restore → same prompt):

build	first request after restore	second
installed b10757 (pin 76dfd64)	prompt_n 1321, cache_n 0, 4342 ms	cache_n 1317, 172 ms
this PR	prompt_n 4, cache_n 1317, 163 ms	cache_n 1317, 162 ms

Server log: saved 3 context checkpoints beside the slot state / restored 3 context checkpoints from the sidecar. Sidecar 197 MB for 3 checkpoints on this hybrid model (~65 MB each: the recurrent state), so checkpoint spacing decides the sidecar size — follow-up in Continuum.