docs: clarify purpose and streamline README entrypoints

[MrScripty]

Make the README a concise entrypoint for Rheon's general 3D fluid-solver and simulation-dataset goal, with native/headless commands and the existing source-bound material-preview image. Keep detailed numerical entrypoints and physical limits in docs/implementation-reference.md.

Cleanup review corrected two stale scope statements before merge: disabling default features omits default PNG export while retaining mandatory dependencies, and the transport-only base step is distinguished from the separately admitted sealed, fully filled Newtonian step_viscous path. No solver, model, scientific receipt, workflow or user data changes.

Validation on exact documentation bytes now committed at 470ce9b6212770bf2abf9fbff4563e19200f561e:

• Independent GPT-6.1 Sol review checked current main d8771ad83c30644d29929c96534548ce238bb357, source/API and feature declarations, commands and existing screenshot provenance.
• All 27 repository-relative links resolve; both Markdown documents render through Pandoc GFM with warnings treated as failures; diff checks pass. These are static/document checks, not new builds, simulations, hardware or model qualification.
• Exact reviewed README SHA256 576954cd220feb32da48a0d76444f890e96d77c9a6a024ec0b1b26f82687bf30; implementation reference SHA256 4b3affc3e034fb455c116a29171692c3fba2c82128637507e1ea17787be255fb. Independent follow-up review SHA256 bf08a3e50b87085fbbfeb7195aa9986f5a968d8c7ca03bbce9addf28ba487.

The shared branch history is preserved: the factual correction is an additive commit after 9dd9ebf. All three exact-head status checks and both hosted workflow runs passed before merge. Merged as bc2f472787060d9a69cb619ed2c2fea10fd754b6; the resulting tree is f875f73279b8bbd4400037db398704040922bc5a, with only the two intended documents changed from prior main. The post-merge scoped check also passed on that merge commit. The measured A1 owner-input blocker is unchanged.

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