Remove dead modules, dead code in showcase, and stale test helpers

[meszmate]

Summary

Audit pass for code that has no callers anywhere — files were imported nowhere, helpers were defined but never called, computed values were explicitly discarded. Net -584 lines, no behavior change, no public API removed beyond the two dead modules.

What was removed

File	Why
`src/style/renderer.zig` (177 lines)	Defines `RenderContext`, `stripAnsi`, `hasAnsi`, `truncate` — zero references in src, examples, or tests. Six test files that needed an ANSI stripper each rolled their own rather than reusing this one.
`src/testing/record_replay.zig` (372 lines)	`Recorder` / `Player` for TUI session capture. No example, no test, no other src file uses it. Speculative API that didn't earn its place. (4 root.zig re-exports also dropped.)
`examples/showcase.zig` (~25 lines)	`renderVerticalBars()` built a bar chart whose result was thrown away by an explicit `_ = vertical_box;` discard line. The helper, the call site, and the dead binding all ran every frame for no visible output.
`tests/markdown_tests.zig` (6 lines)	Orphaned `renderMd` helper at the top of the file: never called, plus a latent use-after-free (returned slice from an arena destroyed on function return).

Audit notes — what was not removed

• `transforms` namespace and `StyleRange` / `renderWithRanges` / `renderWithHighlights` — unused inside the codebase but documented in README, so they're intentional public API. Left alone.
• The `*.zig` example files (e.g. `text_editor`, `focus_form`, `dashboard`) that overlap with `showcase` tabs — kept as small focused single-component demos. They're better learning entry points than the 1000-line showcase.
• All other root.zig re-exports — convenience, low cost, real users.

Test plan

• `zig build` passes
• `zig build test` — full suite passes
• All 44 example binaries still build