[LabDennis]

Long simulation seeks collected every 15-step snapshot before eviction. The final one-time thinning could still leave more than the intended 256 checkpoints, and peak state storage grew with the entire seek distance.

Refs #1238: addresses checkpoint accumulation and the retained-count limit.

One insertion helper now enforces the 256-checkpoint limit both while generating snapshots and while merging them into a cache entry. It drops off-grid targets first, thins aligned snapshots when needed, and keeps the new exact target. The simulator still executes the same steps and returns the same state; repeated requests for the latest target reuse its Arc directly.

Validation on Windows / Rust 1.95, upstream 549e9d82:

• Both new regressions fail with the original production code because the retained checkpoint count exceeds 256.
• All 457 Effects tests pass, including particle/simulation image hashes. The final cache tests and strict all-target Effects Clippy also pass.
• An instrumented small synthetic state verifies retained counts and a peak bound of 514 live states across sequential warm-cache seeks (up to 256 old snapshots + 256 fresh snapshots + working state/pending clone). No large particle buffers or memory-exhaustion tests are needed.
• Forward/backward seeks, repeated targets and key eviction match a fresh deterministic simulation.
• Formatting, assets, layers and the full WASM gate (32 crates, one existing UI dead-code warning) pass. Full xtask ci stops at the unchanged collapsible_match finding in crates/model/src/stl.rs:179; this PR does not claim a green full workspace CI run.

The limit counts snapshots, not bytes: variable-sized states and concurrent runs still need a broader memory-budget design. Very large advances still execute synchronously and are not cancellable. Those remaining parts of #1238 are deliberately not marked closed by this focused fix.

Upstream main, issue status and open PRs were checked before publication. Existing broad performance PR #856 changes plane transposition in the same source file but does not touch SimCache; #885 also does not provide this checkpoint fix.

The branch is now based on upstream 05955c08b4fd. History: 1 → 1 commits. The rewritten file tree was checked against the previous PR plus current upstream, preserving prior integration resolutions.

Scoped Windows regression checks: 1 passed, 0 ignored, zero failures. Formatting, locked Cargo metadata, asset and layer checks passed. This is scoped validation, not a new full-workspace or cross-platform CI claim.