Contribution Proposal: Add stable softplus, mish, and logsumexp conversion support

## What I want to contribute

Add numerically stable ATen-to-Core resolver entries for `softplus`, `mish`, and `logsumexp`. These operations use `exp(x)` internally, which overflows in fp16 when x > ~11.09 (IEEE 754 half-precision max = 65504, ln(65504) = 11.09).

The existing `replace_log_softmax` in `_aten_to_core.py` (line 2541) already implements the stable max-shift decomposition. The proposed additions follow the same pattern.

## Proposed decompositions

| Operation | Naive (unsafe) | Stable (proposed) |
|-----------|---------------|-------------------|
| softplus | `log(1 + exp(x))` | `max(x, 0) + log(1 + exp(-abs(x)))` |
| mish | `x * tanh(log(1 + exp(x)))` | `x * tanh(stable_softplus(x))` |
| logsumexp | `log(sum(exp(x)))` | `max(x) + log(sum(exp(x - max(x))))` |

The softplus decomposition ensures `exp(-abs(x))` is bounded in (0, 1], making overflow impossible in any precision.

## Context

These are the same decompositions being contributed to `coremltools` in PRs [#2725](https://github.com/apple/coremltools/pull/2725), [#2726](https://github.com/apple/coremltools/pull/2726), and [#2727](https://github.com/apple/coremltools/pull/2727). The underlying fp16 overflow issue is documented in the Orion paper (arXiv:2603.06728) and affects any model using these activations on Apple Neural Engine.

## Open question

Before writing code, I need to verify whether `torch.export` with `run_decompositions()` preserves these ops as ATen entries or decomposes them into primitives. If they are decomposed, a `remove_decomps` entry or a graph rewrite pass would be needed instead.

## Scope

- Add 3 resolver functions in `_aten_to_core.py` (~50 lines total)
- Add corresponding dispatch table entries
- Add tests in `tests/ops/test_ops.py` following the existing pattern (e.g., `test_hardswish`, `test_log_softmax`)

Start by checking how torch.export with run_decompositions() represents softplus, mish, and logsumexp. Read the existing replace_log_softmax entry around line 2541 in _aten_to_core.py, then compare the test patterns for test_hardswish and test_log_softmax in tests/ops/test_ops.py. Done means stable resolver functions, dispatch entries, and corresponding tests for all three operations.

Written by the indexing model from the issue text.