Adds a guide to running a statement program

[johnnyt]

What

A how-to page, docs/guides/how-to-run-a-statement-program.md: compile a renewal script with compileProgram, execute it for a loan with execute and read back the loan it leaves, ask executeValue for the last expression's value, and handle a script that fails. Every example is in the library loan.

The README's Documentation map links the page under Do, in place of the "Statement programs" line that pointed the Do reader at a reference section. That line moves to Look up with a description of what the section holds, the shape the host functions guide took. No other README text changes, and no file under src/ changes.

Source

The docs audit names no span for this subject (its next action for this repository is about the Install section), so the page is written from the README's "Statement programs" reference section and the doc comments on compileProgram, execute and executeValue, and that section stays where it is.

Title

The title "How to run a statement program" is the one the bead's acceptance dictates, kept as given. "run" in it is a verb; it is the page's only use of the word.

Result lines

The suite does not read docs/guides/, so no result line on the page is executed by the suite. Each one was executed by hand against src/ on this branch, with assertions, before the commit:

• compiled.ok is true
• compiled.instructions starts ["load", "loan"], ["access", "renewals"], ["lit", 2]
• the refused script answers "unassignable_location" at { line: 1, column: 19 }
• the renewed loan's context is { loan: { renewals: 2, days_left: 17 } }
• the caller's loan.renewals is still 1
• the refused loan's context carries status: "refused"
• executeValue answers 17
• the loan with no days_left fails as "TypeMismatchError" with reason "add"
• that failure's context is { loan: { renewals: 2 } }

Checks

• Full gate (mise exec -- pnpm run gate) green on this tree; README.md is a gated path, so the gate ran.
• The README linter: README.md: 1170 lines, every part present.
• The page contains no word from the fixture-only domains, and no "run" as a noun.
• No changelog fragment: changelog.d/README.md excludes documentation.