docs(as): document the __<fn>_impl entry seam + fix the Install example

[manoahLinks]

Adds the generated-entry-dispatch section otigen's AS codegen needs as its canonical reference: the __<fn>_impl author seam, the full otigen.toml → AssemblyScript type-mapping table, the index.ts + abort wiring, the signature-drift rules, and how the seam is opted into and out of.

Also corrects two things the Phase 1 consolidation (#18) left stale:

• The Install example imported sload / sstore / emit_event / hash_poseidon2 from @pyde-net/host/assembly. Since refactor(as): consolidate the AS SDK into assemblyscript/ (one package, like go/ and rust/) #18, index.ts re-exports the ergonomic wrappers and the raw externs live behind /assembly/raw, so that snippet fails with Element not found — the first thing a new author would copy. Replaced with both subpaths and why they're split; the replacement was compiled with asc to confirm every name resolves.
• The opening claimed "One file, no runtime code, no external dependencies". All three stopped being true with the borsh / codec / u128 / ctx / calldata / exit / hash modules and the as-bignum dependency, and it contradicted the codecs the new section documents.

Docs only — no code, no ABI, no package surface change.