chore: workspace cleanup after card-theme phase landings

Drops dead deps and stale doc content carried over from the pre-MIT
art swap.

Cargo.toml manifests:
- solitaire_core no longer depends on chrono (no source references it
  since the original sync-payload timestamps moved to solitaire_data).
- solitaire_sync no longer depends on serde_json (the sync types use
  serde-derive with whatever serializer the caller picks; the old
  json-specific helpers were removed earlier).

Cargo.lock pruned by `cargo build` to drop the now-untransitively-
referenced versions.

CREDITS.md redistribution clause: "LGPL and OFL notices" tightened to
"MIT (project + hayeah card art) and OFL (FiraMono)" since the LGPL
art is gone.

SESSION_HANDOFF.md:
- HEAD bumped to 924a1e2; test count to 960; 9 ignored.
- Punch list rewritten — the xCards-URL line is obsolete (we did the
  swap), v0.1.0 tag exists locally, and player smoke-test is the
  current top item.
- New "Card-theme system (CARD_PLAN.md, fully shipped)" section
  summarises the seven-phase end-to-end flow so a future session has
  the integration map without re-reading the plan.
- Optional list gains the SVG-vs-layout aspect-ratio note as a
  cosmetic-only follow-up.

Removed the locked worktree at .claude/worktrees/agent-aa55a94d18c669d70
left behind by a prior Claude session.

cargo build / clippy --workspace --all-targets -- -D warnings / test
--workspace all green (960 passed, 0 failed, 9 ignored).

solitaire_core/Cargo.toml

@@ -6,6 +6,5 @@ edition.workspace = true
 
 [dependencies]
 serde     = { workspace = true }
-chrono    = { workspace = true }
 thiserror = { workspace = true }
 rand      = { workspace = true }