fix(loop): replace catch unreachable with try in self_hosting_loop.zig init

[gHashTag]

Summary

• Replace catch unreachable with try for ArrayList initialization in SelfHostingSession.init() (lines 199-200)
• Allocation can fail, so proper error propagation is required instead of asserting impossibility

Test plan

• zig ast-check src/tri/self_hosting_loop.zig passes
• No catch unreachable on ArrayList initCapacity in the file

Closes #159

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