fix: prevent infinite loop in handle_metadata_vars when variable references cycle

[jespo2021]

What

handle_metadata_vars hangs indefinitely when find_replace_values produces a cycling sequence of values (e.g. local.A → local.B → local.A → ...).

The existing guard only breaks when value == old_value (a fixed point). Cycles are different: the value changes each iteration and never repeats consecutively, so the break is never reached.

Root cause

while ("local." in value or "var." in value or ...):
    old_value = value
    value = find_replace_values(value, mod, tfdata)
    if value == old_value:  # only catches fixed points, not cycles
        break

If find_replace_values expands local.A to something containing local.B, then local.B back to something containing local.A, the loop runs forever with no output.

Fix

Add a _seen_values set that tracks every value encountered in the current resolution pass. If the same value appears again, we've detected a cycle — emit the existing "unresolved references remain" warning and break.

_seen_values: set = set()
while (...):
    if value in _seen_values:
        # cycle detected
        click.echo(WARNING...)
        break
    _seen_values.add(value)
    ...

How I found it

Running terravision draw against a Terraform project that uses a Cognito module with a local.lambda_arn map. The module defines optional lambda ARN locals that are conditionally populated — when the lambda isn't configured, the reference chain cycles instead of resolving to a terminal value. Terravision printed all the "Processing resources" entries, then hung silently forever with 100% CPU.

Tested

Confirmed fix resolves the hang on the reproducing project. The diagram generates successfully after the patch.

[patrickchugh]

@jespo2021 Grateful for the PR. Thanks!