seal: lower grounded payloads to ir

[PerishCode]

Summary

• start carrying minimal Grounded AST payloads into IR
• lower value, stream-view, process-call, stream-flow, and @type.* process conversion shapes
• decode string and text-block literals before Grounded/IR
• harden parser recovery for invalid braced stream bindings

Details

This keeps the cold-start Seal pipeline on the agreed boundary: Raw AST preserves source shape, Grounded AST owns semantic payload and metadata, and IR consumes Grounded forms instead of reaching back into Raw AST.

The current @type.* lowering intentionally covers process-call inputs, including block-call sugar like @type.string { | git ... }. Unsupported inputs stay unlowered rather than pretending to be pure calls.

Validation

• cargo fmt
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo test --locked --workspace
• flavor check
• shell syntax checks
• Python syntax checks
• PowerShell syntax checks