Claude review: lib/fonts: Implement font rotation

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Verdict: Good design, minor style nit.**

Introduces `font_data_rotate()` which takes a full font's glyph data and produces a rotated copy. Key design decisions are reasonable:

- `steps % 4` for normalization
- Buffer reuse via `buf`/`bufsize` parameters (avoids reallocation on repeated rotation)
- `check_mul_overflow` for safety
- Returns `ERR_PTR` on failure
- Restores the bulk `memset` optimization (one memset for the entire output buffer, then per-glyph rotation without zeroing)

The `krealloc`-like reuse semantics are well-documented.

**Minor**: The switch for `d_cellsize` has no `default` case. While all values 0-3 are covered after `steps %= 4`, some compilers may warn about potentially uninitialized `d_cellsize`. A `default:` matching case 0/2 would silence this.

The caller-side refactoring in `fbcon_rotate_font` is clean. The error path correctly handles the case where `font_data_rotate` returns ERR_PTR without freeing the old buffer (the caller frees it).

The `fd_size` type change from `u32` to `size_t` is appropriate.

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Generated by Claude Code Patch Reviewer