Clarify v0.3.0 value and restructure documentation

[swgiacomelli]

Summary

• add a concise value statement before the v0.3.0 change inventory
• replace the 1,105-line root README with a 381-line product guide organized from host definition through the runtime measurement pipeline
• add a linked use-case chooser and seven focused quick starts
• add root indexes for docs/, docs/design/, and docs/guides/
• preserve useful detail removed from the landing page in four focused user guides: curve generation, physical transfers, measurement pipelines, and tickless scheduling
• keep exact API, compatibility, schema, and design contracts in their authoritative locations

Documentation structure

• root README: orientation, value, installation, quick starts, guarantees, and API map
• docs/guides/: practical selection, modeling, cost, warm-up, accuracy, and clock guidance
• docs.rs: callable API contracts, errors, panic conditions, and compiled examples
• docs/compatibility.md: release and migration commitments
• docs/design/: rationale, constraints, and historical design decisions
• CHANGELOG: release inventory and the value delivered by v0.3.0

The packaged README uses GitHub links for repository guides because docs/ is intentionally excluded from the crate package.

Trim audit

The detailed material retained outside the root README includes:

• all built-in curves, formula syntax, LUT-size and generated-name constraints
• physical-transfer design steps, source choices, fitting semantics, guards, accuracy scope, and non-goals
• calibration arithmetic, temporal primitive selection and state cost, composition order, warm-up, decisions, and reset ownership
• tickless rollover-safe comparisons, duration bounds, quantization, and repeat behavior

Family schema, host IR internals, compatibility history, and exhaustive API contracts were not duplicated because dedicated design, compatibility, and rustdoc sources already own them.

Validation

• cargo fmt --all -- --check
• cargo test --doc --all-features (4 passed, 5 intentionally ignored)
• cargo package --list --allow-dirty (55 files; package boundary unchanged)
• local link-target and heading-anchor audit
• git diff --check

This PR targets release/0.3.0. It does not merge or replace the aggregate release PR #64.

[chatgpt-codex-connector]

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