test: use isapprox for sparsified VirtualPTDF row comparison

[luke-kiernan]

The exact == compared two VirtualPTDF solver paths that differ by ~1 ULP, making test_auto_tolerance.jl:191 flaky; switch to isapprox.

🤖 Generated with Claude Code

[github-actions]

Performance Results

Precompile Time

Main	This Branch	Delta
2.2 s	2.3 s	+1.7%

Execution Time

Cells show median (min–max) over 5 samples; delta compares medians.

Test	Main	This Branch	Delta
matpower_ACTIVSg2000_sys-Build Ybus	9.4 (9.1–12.1) ms	9.4 (9.2–11.1) ms	+0.1%
matpower_ACTIVSg2000_sys-Build PTDF	74.0 (73.0–74.5) ms	74.2 (73.1–75.4) ms	+0.3%
matpower_ACTIVSg2000_sys-Build LODF	152.8 (150.6–154.3) ms	155.5 (152.7–156.2) ms	+1.8%
matpower_ACTIVSg2000_sys-Build VirtualMODF	92.1 (91.8–92.8) ms	1.0 (1.0–1.0) s	+1029.0%
matpower_ACTIVSg2000_sys-VirtualMODF Query 10 rows	0.6 (0.6–1.4) ms	0.6 (0.6–0.8) ms	-0.3%
matpower_ACTIVSg2000_sys-Radial network reduction	1.0 (0.8–1.8) ms	0.9 (0.9–1.1) ms	-8.2%
matpower_ACTIVSg2000_sys-Degree two network reduction	1.0 (0.9–1.5) ms	0.9 (0.8–0.9) ms	-10.1%
Base_Eastern_Interconnect_515GW-Build Ybus	561.9 (545.1–570.9) ms	513.1 (507.0–521.0) ms	-8.7%
Base_Eastern_Interconnect_515GW-Build VirtualPTDF	782.5 (778.8–788.0) ms	736.3 (732.9–742.6) ms	-5.9%
Base_Eastern_Interconnect_515GW-VirtualPTDF Query 10 rows	24.7 (24.4–25.3) ms	24.6 (24.1–26.0) ms	-0.6%
Base_Eastern_Interconnect_515GW-Build VirtualMODF	785.2 (782.6–790.6) ms	816.4 (815.9–830.9) ms	+4.0%
Base_Eastern_Interconnect_515GW-VirtualMODF Query 10 rows	28.7 (28.5–29.9) ms	29.2 (28.9–30.4) ms	+1.8%
Base_Eastern_Interconnect_515GW-Radial network reduction	36.2 (35.1–36.6) ms	36.9 (34.7–38.3) ms	+2.0%
Base_Eastern_Interconnect_515GW-Degree two network reduction	52.4 (52.0–53.4) ms	47.8 (47.0–56.9) ms	-8.8%