FIX: Memoize Singed.output_directory to prevent path override

[benaitcheson]

Problem

After setting Singed.output_directory = "flamegraphs/" inside development.rb the gem initialisation defaults it back to "tmp/speedscope"

Solution

Added memoization using the ||= operator to cache the computed path after its first calculation. This ensures that:

• The path is only computed once during Rails initialization
• The original fallback behavior is preserved if a custom path is set before initialization

Impact

• Reduces unnecessary path computations
• Maintains compatibility with existing custom path configurations
• No breaking changes to the public API

Testing

• Verified that the output directory is correctly set on initialization
• Confirmed that custom paths set before initialization are not overwritten

Resulting stacktrace from server after change with bundle open

Before hand:

development.rb setup

[benaitcheson]

@technicalpickles Would I be able to get a review on this one please. Thanks.