Add gather / embedding StableHLO converter (#483)

[michalharakal]

Closes #483.

Summary

Adds a real converter for `gather` / `embedding` / `index_select` / `Embedding`. Every LLM export begins with a token-id \u2192 embedding lookup, and without this converter a traced Llama / Mistral / Qwen / Gemma forward pass was failing at its very first operation — it never reached the norms, softmax, RMSNorm, LayerNorm, or attention that the other P1 converters cover.

Target lowering

For the canonical `embedding(input_ids)` shape (vocab_size=8, hidden_size=4, seq_len=3, axis=0):

```mlir
%out = stablehlo.gather(%W, %ids)
{ dimension_numbers = #stablehlo.gather<
offset_dims = [1],
collapsed_slice_dims = [0],
start_index_map = [0],
index_vector_dim = 1>,
slice_sizes = array<i64: 1, 4>,
indices_are_sorted = false }
: (tensor<8x4xf32>, tensor<3xi32>) -> tensor<3x4xf32>
```

The `offset_dims` / `collapsed_slice_dims` / `start_index_map` are derived from the gather axis. `slice_sizes` gets a `1` along the gathered axis and the full extent along every other dimension. Negative axes are normalized against the weight rank.

Registered in `StableHloConverterFactory.createBasic` / `createExtended` / `createFast`.

Two commits

1. Failing test

`GatherConverterTest` with 4 cases:

• Three alias-registration tests (`gather`, `embedding`, `index_select`) asserting each is claimed by a converter and not dropped as unsupported.
• `embedding_lowering_carries_canonical_dim_numbers_and_slice_sizes` asserting the exact `offset_dims = [1]` / `collapsed_slice_dims = [0]` / `start_index_map = [0]` / `slice_sizes = array<i64: 1, 4>` shape for the canonical embedding case, plus a tight operand-shape assertion:
  ```kotlin
  assertTrue(mlir.contains("stablehlo.gather(%arg0, %arg1)"))
  assertFalse(mlir.contains("stablehlo.gather([%"))
  ```
  This catches a Kotlin string-template pitfall I hit in development: `"$operands[0]"` expands to `[a, b][0]` literal, producing `stablehlo.gather([%arg0, %arg1][0], [%arg0, %arg1][1])`. The assertion pins the correct shape so the bug can never regress silently.

2. The converter

`GatherOperationsConverter.kt` implementing the lowering above, plus factory registration.

Test plan

• `GatherConverterTest` — 4/4 green
• `./gradlew :skainet-compile:skainet-compile-hlo:allTests -x kotlinWasmStoreYarnLock` — green across jvmTest, wasmJsTest, wasmJsBrowserTest, wasmWasiTest, wasmWasiNodeTest, macosArm64Test, iosSimulatorArm64Test (`linuxX64Test` skipped on macOS host)
• CI: full multiplatform build

Out of scope

• Higher-rank gathers (attention-side index gathers, multi-dim scatter/gather). Add when a traced model surfaces the pattern.
• Scatter. Separate converter when needed.
• Quantized embedding tables — depends on further P0-1 track work.
• Rotary position embedding (RoPE). Separate issue.

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