consolidate cache commands into `cache` subcommand (#978)

Consolidate the scattered cache-related top-level commands into a single `zen cache <sub>` command tree, keeping the old names as hidden deprecated aliases so any existing scripts keep working.

## Motivation

`zen` has accumulated a flat list of cache-adjacent commands (`cache-info`, `cache-stats`, `cache-details`, `cache-gen`, `cache-get`, `drop`, `rpc-record-start/stop`, `rpc-record-replay`). Each one re-declares `--hosturl` parsing and host resolution, and there is no natural home for new cache tooling. Grouping them under `cache` gives a consistent UX and a shared base class to hang common options off of.

## Changes

### Subcommand consolidation
- Moved into `cache <sub>` form:
  - `cache info`, `cache stats`, `cache details`, `cache gen`, `cache get`, `cache drop`
  - `cache record <path>` / `cache record stop` (formerly `rpc-record-start` / `rpc-record-stop`)
  - `cache replay` (formerly `rpc-record-replay`)
- All old top-level names remain as deprecated aliases and forward through a shared legacy-shim dispatcher that rewrites `argv` and re-enters the new dispatcher, so behavior is byte-identical for existing callers.
- Deprecated aliases are now hidden from the top-level `zen --help` listing (new `ZenCmdBase::IsHidden()` + `DeprecatedCacheStoreCommand` base). They still dispatch normally; `zen cache --help` is the canonical discovery surface.

### Shared base class
- New `CacheSubCmdBase` owns the `--hosturl` option and `ResolveHost()` logic, eliminating the copy/pasted block at the top of every `Run()`.

### Output format
- Added `--yaml` to `cache info`, `cache stats`, and `cache details` (negotiated server-side via `Accept: text/yaml`). `cache details` now rejects `--csv --yaml` combined.

### Hardening
- `cache gen`: bounds-check requested sizes before allocating.
- `cache replay`: validate `--stride` / `--offset` and fix progress-math overflow edge cases.

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