Port upstream 0.63.0: MiniMax checked reset arithmetic (stacked on #651)

[Finesssee]

Summary

Closes the remaining MiniMax half of GAP-17 (audit-0.63.0 Gap 1, partial item #6): guard the MiniMax reset-time arithmetic so an oversized reset value can never abort the refresh. The rest of steipete#3758/steipete#3764 was already covered by #585/#586.

• coding_plan.rs::resets_at: build the offset with Duration::try_seconds (None for an unrepresentable remains, e.g. i64::MAX after the >1_000_000 ms heuristic) and finish with now.checked_add_signed; a reset that lands at/before now is dropped, so the usage percentage survives with no reset line.
• coding_plan_html.rs::parse_resets_at_from_text: "Resets in N " text with a very large N saturates seconds as i64; the same try_seconds + checked_add_signed guard turns it into no reset line instead of a panic.
• Tests: oversized remains = i64::MAX, negative/zero remains, positive small/ms remains, future end-time precedence, "Resets in 99999999999999999999 days" saturation, and a normal 2-day text.

Stacked on #651 (port/micro-0.66.0-minimax-storage-discovery at 084719a8). Backend only, no UI change (a dropped reset line is the intended behavior). No new dependencies.

Validation

• cargo fmt --all --check: pass
• cargo clippy both manifests --all-targets -- -D warnings: clean except the 3 documented pre-existing main-drift sites
• cargo test rust manifest: 2306 passed / 0 failed / 1 ignored (minimax coding-plan focused: 19/0 incl. the new overflow tests)
• cargo test desktop manifest: 489 passed / 1 failed — bootstrap_payload_exposes_every_provider_variant, the documented Isolate bootstrap payload test from real settings #684 environment-dependent baseline on branches without Make the bootstrap catalog test hermetic (#684) #711 (expected)

Upstream reference

• Release bullet: "Providers: ... safely handle unrepresentable timestamps (fix(providers): avoid crashes on oversized usage values steipete/CodexBar#3758, fix(providers): handle oversized counts and reset dates steipete/CodexBar#3764)", v0.63.0, commits d987dd59a / 70dfb8f1b; audit-0.63.0.md Gap 1. Upstream Swift Date(timeIntervalSinceNow:) does not trap; this is the Rust analog.

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[Finesssee]

Adversarial validation (lane-A) at cd00d98

Implementation matches audit-0.63.0.md Gap 1 exactly: oversized remains / "Resets in N days" text can no longer abort the refresh; the offset is built with Duration::try_seconds and applied via now.checked_add_signed, returning no reset line and keeping the usage percentage. Tests cover remains = i64::MAX, negative/zero remains, small/ms remains, future end-time precedence, the 99999999999999999999-days saturation case, and a normal 2-day text (6 new tests).

Checks at cd00d98 (CARGO_TARGET_DIR=W:\cargo-target\lane-a, jobs=4, RUST_TEST_THREADS=4): fmt pass; clippy both manifests -D warnings (3 documented pre-existing main-drift sites only, 0 in diff); rust tests 2306/0/1-ign; desktop tests 489/1 (documented #684 #711 baseline, expected).

Stacked on #651 at 084719a; backend only, no UI proof required.

[Finesssee]

Shipped in v0.70.0: this PR's head is included in main via #735 (merge commit 9d0a37a). Closing as integrated.