Rename notebooks/ → livebooks/ - #178

Extend `mode_test/2` to `mode_test/3` with a `lane_tags:` option and run
the `*_full` forward-pass conformance suites (ViT-Base, Whisper-tiny)
through the native and fusion compilers alongside the evaluator.

The full suites pass `lane_tags: false`, so their native and fusion
lanes are emitted without the cross-cutting `:native` /
`:native_compiled` tags and stay gated behind the suite's own `:vit_full`
/ `:whisper_full` moduletag. `--only vit_full` now runs all three lanes
on the full checkpoint, while `--only native` stays tiny-random only and
never pulls a full-size download.

Both full forwards lower fully under `native_fallback: :raise` and hold
at the pinned 1e-4 reference tolerance in the fusion lane too —
`mx::compile` only reassociates f32, so its drift stays well below the
approximate `mx::fast::*` kernels (which need 1e-3 ViT / 1e-2 Whisper in
the `:fast_kernels_full` variants). The serving and fast-kernels tests
remain eval-only.

`Nx.take_along_axis` (the `Nx.Block.TakeAlongAxis` block) now lowers
under `compiler: Emily.Compiler, native: true` instead of raising. The
MLX op and eager NIF already existed (`Emily.Native.take_along_axis` →
`mlx::core::take_along_axis`, used by `Emily.Backend.native_take_along_axis/4`);
only the IR path was missing.

- ir.ex: add opcode `take_along_axis` (81) and a `lower_block/5` clause
  mirroring the eager twin — cast indices to s32, emit, coerce to output
  type — alongside the existing `Nx.Block.Take` lowering.
- opcodes.hpp: add `TakeAlongAxis = 81`, bump `kOpcodeCount`, and a
  dispatch case calling `mx::take_along_axis(in[0], in[1], axis, s)`.
- compiler_equivalence_test.exs: native-vs-evaluator bit-identical cases
  (last axis, axis 0, a 3-D transformer-shaped gather).

This was the last op forcing a fallback in
`Bumblebee.Text.question_answering`'s answer-span gather, so the
DistilBERT-QA `Nx.Serving` forward now compiles fully native and fused.
Wire it into the conformance suite: extend `mode_test` with a `:tag`
option (stamp every lane) so the serving test runs its eval / native /
native_compiled lanes gated behind `:distilbert_full`.

The opcode wire values are hand-maintained in two places —
`Emily.IR`'s `@opcodes` map and the `Opcode` enum + `kOpcodeCount` in
`c_src/emily/opcodes.hpp` — with nothing enforcing they agree. A
mismatch compiles fine and only misbehaves at runtime.

Add `Emily.OpcodeParityTest`, which parses the header and asserts both
sides are a unique, gap-free `0..N-1` with N == `kOpcodeCount`. The
check is value-based, not name-based, so it doesn't depend on the
snake_case names matching the PascalCase enum (`negate`/`Negative`,
`fast_rms_norm`/`FastRMSNorm`); a name/value permutation that keeps both
contiguous is still caught by the equivalence suite. Expose
`Emily.IR.opcodes/0` for the test to read the map.

The example documents are Livebooks (`.livemd`), so name the directory
to match. Rename `notebooks/` → `livebooks/` (all 8 files) and update
every current-state reference:

- mix.exs: the `extras:` paths and the `groups_for_extras` regex, plus
  the nav-group label `Notebooks:` → `Livebooks:`.
- scripts/test-livebooks.sh: `NB_DIR` and the path comment.
- README.md / ROADMAP.md: the `livebooks/` path and the HexDocs
  "Livebooks" section references.

CHANGELOG.md and PLAN.md keep their `notebooks/` mentions — those are
point-in-time historical records, not current-structure docs.