docs(install): the Cargo feature table omits three declared features

## Summary

The Cargo feature table in `docs/installation.md` covers 7 of the 10 declared features. The three missing ones are user-reachable (they gate examples and a parity test), and each already has a Cargo.toml comment that condenses into a table row.

## Background

Because the table is incomplete, a user cannot learn from the docs that these features exist or what they gate. Each feature already carries an explanatory comment in `Cargo.toml`, so the rows write themselves.

## Proposed Solution

Add three rows condensed from the existing Cargo.toml comments, and optionally add the one missing example comment.

## Implementation Notes

- Table: `docs/installation.md:27-36`.
- Missing features (confirmed in `Cargo.toml`): `xla-diagnostics` (`:103`), `xla-micro-oracle` (`:106`), `xla-diagnostics-cpu` (`:110`).
- Reachability: the first two gate `[[example]]` targets (`Cargo.toml:412`, `:419`, `:430`); the third gates `tests/molmo2_xla_vision_parity.rs`.
- Minor companion: `[[example]] xla_aux_smoke` (`Cargo.toml:421-423`) is the only example block without an explanatory comment; every sibling has one.

## Acceptance Criteria

- [ ] Every declared feature appears in the installation feature table.

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## Original Suggestion

### Title: docs(install): the Cargo feature table omits three declared features

The Cargo feature table in `docs/installation.md` covers 7 of the 10 declared features; the three missing ones are user-reachable (they gate examples and a parity test), and each already has a Cargo.toml comment that can be condensed into a table row.

## Evidence

- Table: `docs/installation.md:27-36`
- Missing: `xla-diagnostics` (`Cargo.toml:103`), `xla-micro-oracle` (`:106`), `xla-diagnostics-cpu` (`:110`)
- Reachability: the first two gate `[[example]]` targets (`Cargo.toml:412`, `:419`, `:430`); the third gates `tests/molmo2_xla_vision_parity.rs`

Minor companion in the same area: `[[example]] xla_aux_smoke` (`Cargo.toml:421-423`) is the only example block without an explanatory comment; every sibling (`:386-:433`) has one.

## Suggested fix

Add three rows condensed from the existing Cargo.toml comments, and optionally the one missing example comment.

## Acceptance criteria

- [ ] Every declared feature appears in the installation feature table

Start with the feature table in docs/installation.md:27-36 and compare it with the comments for xla-diagnostics, xla-micro-oracle, and xla-diagnostics-cpu in Cargo.toml:103-110. Condense those comments into three table rows, optionally document xla_aux_smoke at Cargo.toml:421-423, and verify that every declared feature appears in the installation table.

Written by the indexing model from the issue text.