feat: add colorgrad, transcript parser, terminal palette detection, and expanded color/input systems

Infrastructure layer for the TUI visual overhaul. Introduces foundational
modules and capabilities that the section-level features build on:

colorgrad (0.7) dependency:
  OKLab gradient interpolation for per-character color transitions in
  sparklines and context bars. Adds ~100K to binary (929K -> 1.0M).

color.rs expansion:
  - parse_hex(): #RRGGBB and #RGB -> (u8, u8, u8) conversion
  - fg_rgb()/bg_rgb(): 24-bit true-color ANSI escape generation
  - gradient_fg(): two-point interpolation via colorgrad
  - make_gradient()/sample_fg(): multi-stop gradient construction and sampling
  - resolve_color() now supports: hex (#FF6B35), bg:color, bg:#hex,
    italic, underline, strikethrough, and palette refs (p:success)
  - Named background constants (BG_RED through BG_WHITE)

transcript.rs (new module):
  Parses Claude Code transcript JSONL files to derive tool use counts,
  turn counts, and per-tool breakdowns. Claude Code doesn't include
  total_tool_uses or total_turns in its JSON — we compute them by scanning
  the transcript. Includes compact cache serialization format and
  skip_lines support for /clear offset handling.

terminal.rs (new module):
  Auto-detects the terminal's ANSI color palette for theme-aware tool
  coloring. Priority chain: WezTerm config > Kitty config > Alacritty
  config > OSC 4 escape sequence query. Parses Lua (WezTerm), key-value
  (Kitty), and TOML/YAML (Alacritty) config formats. OSC 4 queries
  use raw /dev/tty I/O with termios to avoid pipe interference. Includes
  cache serialization helpers for 1-hour TTL caching.

input.rs updates:
  - All structs now derive Serialize (for --dump-state diagnostics)
  - New fields: transcript_path, session_id, cwd, vim.mode, agent.name,
    exceeds_200k_tokens, cost.total_api_duration_ms
  - CurrentUsage: added input_tokens and output_tokens fields
  - #[serde(flatten)] extras on InputData and CostInfo for forward compat

cache.rs:
  Added flush_prefix() for /clear detection — removes all cache entries
  matching a key prefix (e.g., "trend_" to reset all sparkline history).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -23,6 +23,7 @@ unicode-segmentation = "1"
 libc = "0.2"
 serde_path_to_error = "0.1"
 serde_ignored = "0.1"
+colorgrad = "0.7"
 
 [dev-dependencies]
 criterion = { version = "0.5", features = ["html_reports"] }