docs: comprehensive reference documentation for all SPEL components

[jimmy-claw]

Document every public-facing component of SPEL.

Scope

Macros

• #[lez_program] — all attributes, generated code, constraints
• #[instruction] — account validation attributes
• generate_idl!

Framework types

• Account, AccountWithMetadata, AccountPostState, ChainedCall
• LezError, LezResult, LezOutput

CLI

• Every subcommand: deploy, run, pda, inspect (feat: spel inspect — fetch and decode account data into human-readable output #60)
• All flags and examples

IDL format

• Full schema, discriminators, all fields

FFI / client-gen

• How to run lez-client-gen, what it generates
• Using generated clients in Qt/C++

Approach

1. Inventory all source first
2. cargo doc for API reference
3. Human guides in docs/
4. GitHub Pages via CI

Tracks with #61 (tutorial).