feat(evolve-kernel): kernel source/diff viewer in detail page

[6eanut]

The detail page could open a library kernel's code, but the code was rendered as a plain

 with no line numbers and no way to see what a kernel actually changed relative to a baseline — the key question when reviewing an evolved kernel is "what did it change, vs the original or vs the kernel it was derived from?".

Add a diff endpoint and an inspector panel:

_state_readers.py: read_kernel_source() resolves one library kernel by id; read_kernel_diff() produces a unified diff against the original reference kernel or against the kernel's parent_id. Missing kernels, a root kernel with no parent, and an evicted parent (library caps at MAX_LIBRARY_SIZE) return exists=false with a reason instead of raising.
routes.py: GET /kernels/{kernel_id}/diff?base=reference|parent.
ok-runtime-api.js: getKernelDiff() client.
ok-detail.js: KernelInspector panel with Source / Diff vs reference / Diff vs parent modes, a line-numbered source renderer, and a colored unified-diff renderer with +/- stats. The parent mode is disabled when the kernel has no parent.
ok.css: .ok-source* / .ok-diff* styles.
test_kernel_diff.py: 11 tests covering both readers and their non-raising failure paths.