Fix crash when importing some builds to party tab

[unrealdreamz]

Summary

• Fix party member stat exports that contain decimal metadata values such as MovementSpeedMod|percent|max=129.6.
• Add regression coverage for decimal stat metadata and multi-key nested stat exports.

Root Cause

• PartyTab.ParseBuffs treated any stat export line containing . as a nested key.subkey=value line.
• Decimal values also contain ., so max=129.6 was sent through the nested-stat path.
• That path then attempted to index a nil nested label table and crashed during party import.

Fix

• Replace the broad dot check with an anchored nested-stat matcher: key.subkey=value.
• Only use the nested path when the whole line matches that shape.
• Ensure the nested table exists before assigning subkeys.
• Decimal metadata now stays on the normal stat-value parsing path.

Validation

• PASS: git diff --check
  • Output only included line-ending warnings from the Windows checkout.
• PASS: focused Lua reproduction of the root cause/fix:
  • old broad dot path crashes on MovementSpeedMod|percent|max=129.6
  • anchored nested-stat path parses decimal metadata and preserves nested stats
• PASS: targeted Busted spec after installing the repo's LuaJIT/Busted dependencies locally:
  • Command: busted --lua=luajit ../spec/System/TestPartyTab_spec.lua
  • Output: 2 successes / 0 failures / 0 errors / 0 pending : 4.297 seconds
• Not run locally: docker-compose up
  • Docker/docker-compose are not available in this local environment.

Risk / Rollback

• Low risk: this narrows an overly broad parser branch to the nested-stat syntax it was meant to handle.
• Valid nested exports remain covered by the regression test.
• Rollback is the single parser change plus the test file.

Fixes #1689