… pipeline

Reduce per-frame work in the rendering pipeline after the architectural
changes in #135, #136, and #137:

Rendering pipeline refinement:
- DifferentialRenderer accepts terminal dimensions from the caller,
  eliminating redundant terminal::size() syscalls on every frame
- Replace FNV-1a full-content hashing with lightweight length-based
  identity check; per-line comparison already catches all differences
- Only flush stdout when actual terminal writes occurred
- Track change state to skip no-op flush operations

Terminal housekeeping reductions:
- Hide cursor once at session start instead of per-frame Hide/Show churn
- Pass resize dimensions directly to DifferentialRenderer via
  update_dimensions() on resize events
- Remove per-frame stdout acquisition for cursor operations

Hot-path allocation cleanup:
- print_colored_text: zero-allocation fast path when width is None
  (the common case), avoiding text.to_string() on every call
- truncate_to_width: returns Cow<str> with zero-copy borrow when the
  string already fits, with an ASCII fast-path for byte-level slicing
- Process renderer: reuse RowFormatter scratch buffer across rows,
  use write! into pre-sized buffers instead of per-field format! calls,
  borrow command string instead of cloning, use Cow<str> for GPU percent
  and GPU memory fields, reuse a single clear-line string for padding
- draw_bar/draw_bar_multi: batch consecutive same-type segments into
  single print_colored_text calls instead of per-character emission,
  reducing terminal escape sequence count by ~50x per gauge
- format_cpu_time: return Cow<str> to avoid allocation for common
  "0:00:00" case
- format_hostname_with_scroll: build scroll window directly into a
  pre-sized String without intermediate extended-hostname allocation
- BufferWriter: add reset() method for future frame-to-frame reuse

Measurement support:
- Add unit tests for BufferWriter throughput, DifferentialRenderer state
  management, and truncate_to_width allocation behavior
- Add timing-based throughput tests for the render path

…nfinite loop

- DifferentialRenderer: remove dead content_bytes fast-check that never
  short-circuited; the per-line comparison is already the authoritative
  change-detection mechanism and is O(1) for identical strings
- format_hostname_with_scroll: add ASCII guard before byte-level indexing
  to prevent character corruption on non-ASCII hostnames
- draw_bar_multi: guarantee forward progress in segment batching loop
  when text overlay is behind current position

Add test coverage for the optimized code paths introduced in this PR:
- process_renderer: write_memory_size correctness, format_cpu_time Cow
  allocation behavior (zero/long-running borrows vs owned), RowFormatter
  scratch buffer reuse, and throughput measurements for the zero-seconds
  hot path
- widgets: draw_bar and draw_bar_multi correctness (label, brackets,
  percentage text, custom text, long-label trimming) and escape-sequence
  batching verification confirming the ~50x reduction in terminal writes
  per gauge; includes throughput measurements for both functions