v0.1.7 Major UX Overhaul

ledgr 0.1.7

Breaking changes

• Began the v0.1.7 experiment-first API reset. The public research workflow now
  centers on ledgr_experiment() and ledgr_run() rather than db_path-first
  calls and direct ledgr_backtest() usage.
• The v0.1.7 strategy contract is function(ctx, params). Strategies without
  tunable parameters receive params = list().
• The v0.1.7 context target constructors are ctx$flat() and ctx$hold();
  the older ctx$targets() and ctx$current_targets() helpers now fail with
  migration guidance.

New features

• Added ledgr_opening() for explicit opening cash, positions, and optional
  cost basis.
• Added ledgr_experiment() as the central object for the experiment-first
  workflow.
• Added ledgr_run() as the public single-run API for ledgr_experiment
  objects, including run-time features = function(params) evaluation and an
  explicit seed = NULL identity field.
• Converted experiment-store APIs to snapshot-first signatures and extended
  ledgr_snapshot_load() so a durable file with exactly one sealed snapshot can
  be resumed without retyping the snapshot id.
• Added ledgr_param_grid() as a typed, non-executing parameter-grid object
  with stable canonical-JSON labels for future sweep/tune workflows.
• Added curated print methods for ledgr_run_list() and
  ledgr_compare_runs() while keeping the underlying objects tibble-compatible.
• Added ledgr_demo_bars and ledgr_sim_bars() as deterministic offline demo
  data for examples and documentation.
• Made durable ledgr_backtest handles safer to clean up: explicit close(bt)
  checkpoints before disconnecting, and a finalizer safety net attempts one
  auto-checkpoint if a durable handle is garbage-collected without close.
• Rewrote README and vignettes around the v0.1.7 experiment-first workflow and
  added a v0.1.6 to v0.1.7 migration guide.
• Added ledgr_opening_from_broker() as a reserved adapter hook. v0.1.7 does
  not ship built-in broker integrations.