fix: use merge semantics for Conda GlobalFiltered sync (Fixes #400)

[karthiknadig]

Conda::sync_refresh_state_from used clear+replace for both Full and GlobalFiltered(Conda) scopes. A GlobalFiltered refresh should not drop environments discovered by a previous full refresh.

• Full scope: clear+replace (unchanged)
• GlobalFiltered(Conda) scope: merge via insert_many without clearing
• Stale entries may linger until the next full refresh, which is the safer tradeoff
• Added 4 tests covering full replace, filtered merge, Full→Filtered roundtrip, and irrelevant-scope no-op

Fixes #400

[github-actions]

Performance Report (Linux) ➖

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta	Change
Server Startup	1ms	1ms	1ms	0ms	0%
Full Refresh	96ms	146ms	78ms	18ms	20.0%

Results based on 10 iterations. P50 = median, P95 = 95th percentile.

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Legend

• 🚀 Significant speedup (>100ms faster)
• ✅ Faster than baseline
• ➖ No significant change
• 🔺 Slower than baseline (>100ms)
• ⚠️ Significant slowdown (>500ms)

[github-actions]

Test Coverage Report (Linux)

Metric	Value
Current Coverage	73.4%
Base Branch Coverage	73.4%
Delta	0% ➖

---

Coverage unchanged.

[github-actions]

Performance Report (macOS)

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta
Server Startup	49ms	467ms	80ms	-31ms
Full Refresh	93ms	25650ms	146ms	-53ms

Results based on 10 iterations. P50 = median, P95 = 95th percentile.

---

Legend

• 🚀 Significant speedup (>100ms faster)
• ✅ Faster than baseline
• ➖ No significant change
• 🔺 Slower than baseline (>100ms)
• ⚠️ Significant slowdown (>500ms)

[github-actions]

Test Coverage Report (Windows)

Metric	Value
Current Coverage	69.65%
Base Branch Coverage	69.61%
Delta	0.04% ✅

---

Coverage increased! Great work!

[github-actions]

Performance Report (Windows) ✅

Metric	PR (P50)	PR (P95)	Baseline (P50)	Delta	Change
Server Startup	9ms	12ms	9ms	0ms	0%
Full Refresh	148ms	394ms	150ms	-2ms	-1.3%

Results based on 10 iterations. P50 = median, P95 = 95th percentile.

---

Legend

• 🚀 Significant speedup (>100ms faster)
• ✅ Faster than baseline
• ➖ No significant change
• 🔺 Slower than baseline (>100ms)
• ⚠️ Significant slowdown (>500ms)