docs(readme): reframe as a compiler for system-architecture models

[avrabe]

What

Spar has outgrown the "AADL toolchain" label. AADL is now one of several front-ends (alongside SysML v2 / KerML and CAN .dbc), and the network-calculus timing + verification middle-end is a substrate any architecture formalism can feed. This PR rewrites the README (and the GitHub About, set separately) to reflect that — positioning spar as a compiler for system-architecture models.

Changes

• Tagline + badges — new framing; added SysML v2 and TSN / Network Calculus badges.
• Rewritten intro — explicit compiler structure: front-ends → IR (salsa HIR) → middle-end (analysis + network calculus) → back-ends (codegen + diagrams).
• New ## Components Mermaid graph — all 23 crates grouped by compiler stage with their dataflow, plus the hand-off to the PulseEngine chain (witness · sigil · smithy · loom).
• CLI table — refreshed to the 16 documented subcommands (emit, codegen, dbc, moves, insight, mcp added; modes description corrected).
• Architecture section — reorganized by compiler stage; 20 crates → 23 crates.
• Key Features — multi-formalism ingest and network-calculus timing added; stale 1.3 MB figure dropped.

Verification

Every crate name, subcommand, and count in the new text was checked against the source tree:

• 23 crate dirs under crates/ match the Components graph and Architecture list exactly.
• All 16 table subcommands exist in the spar-cli dispatch.
• Mermaid subgraph/end pairs balanced (5/5).

Docs-only change — no code, no Rust formatting impact.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

Rivet verification gate

✅ 20/20 passed

	count
Passed	20
Failed	0
Skipped (no steps)	0

Filter: (and (= type "feature") (or (has-tag "v093") (has-tag "v0100")))

Failed artifacts

(none)

_{Updated automatically by tools/post_verification_comment.py. Source of truth: artifacts/verification.yaml.}