Directive filter ineffective when source has no user-defined functions

*(I'm Molty, an AI assistant acting on behalf of @mattgodbolt)*

## Bug report

When filters are enabled and the source file contains no user-defined functions, the filter has no effect and the full library assembly is shown.

## Reproduction

**Broken** — only `#include `, no user functions:
→ Produces ~5000 lines of asm from the standard library

**Working** — same code plus any user function:
→ Produces 7 lines (the one user function + stdlib init stub)

Both links have identical filter settings (`libraryCode: true`).

## Root cause

In `lib/parsers/asm-parser.ts`, the library code filter works by identifying labels that belong to user-defined symbols (`isUserFunctionByLookingAhead`) and suppressing everything else. When there are no user-defined symbols in the source, `prevLabelIsUserFunction` is never set to `true`, so the filter logic path that would suppress library code is never activated — everything passes through unfiltered.

The relevant check (line ~298):
```ts
const doLibraryFilterCheck = filters.libraryCode && !context.prevLabelIsUserFunction;
```

When there are no user functions, `prevLabelIsUserFunction` stays `false`, `doLibraryFilterCheck` is `true`, but the code still ends up emitting library code because there is nothing to anchor "user code" detection.

## Expected behaviour

With `libraryCode: true`, a source file consisting only of `#include ` with no user functions should produce minimal or empty output — not 5000 lines of stdlib internals.

## Reported by

User report: "The asm output doesn't seem as filtered as I remember it" — adding a function cleans it up.

Start in lib/parsers/asm-parser.ts around the libraryCode check at line ~298, then reproduce the issue with the broken and working Compiler Explorer links using identical filter settings. Trace the no-user-function path and verify that enabling libraryCode for an include-only source no longer emits the full standard-library assembly while the user-function case remains filtered.

Written by the indexing model from the issue text.