chore: modernize CI and update Go toolchain

- Bump Go from 1.19 to 1.26 and update all dependencies
- Rewrite CI workflow with matrix strategy (Linux, macOS, Windows)
- Update GitHub Actions to current versions (checkout@v4, setup-go@v5)
- Update CodeQL actions from v1 to v3
- Fix cross-platform bug in mock/path.go (path.Join -> filepath.Join)
- Clean up dependabot config (weekly schedule, remove stale ignore)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/github.com/pjbgf/sha1cd/ubc/doc.go

@@ -1,3 +0,0 @@
-// ubc package provides ways for SHA1 blocks to be checked for
-// Unavoidable Bit Conditions that arise from crypto analysis attacks.
-package ubc

vendor/github.com/pjbgf/sha1cd/ubc/ubc.go

@@ -1,8 +1,10 @@
+// ubc package provides ways for SHA1 blocks to be checked for
+// Unavoidable Bit Conditions that arise from crypto analysis attacks.
+package ubc
+
 // Based on the C implementation from Marc Stevens and Dan Shumow.
 // https://github.com/cr-marcstevens/sha1collisiondetection
 
-package ubc
-
 type DvInfo struct {
 	// DvType, DvK and DvB define the DV: I(K,B) or II(K,B) (see the paper).
 	// https://marc-stevens.nl/research/papers/C13-S.pdf
@@ -21,10 +23,12 @@ type DvInfo struct {
 	Dm [80]uint32
 }
 
-// CalculateDvMask takes as input an expanded message block and verifies the unavoidable bitconditions
-// for all listed DVs. It returns a dvmask where each bit belonging to a DV is set if all
-// unavoidable bitconditions for that DV have been met.
-// Thus, one needs to do the recompression check for each DV that has its bit set.
+// CalculateDvMask takes as input an expanded message block and
+// verifies the unavoidable bitconditions for all listed DVs. It returns
+// a dvmask where each bit belonging to a DV is set if all unavoidable
+// bitconditions for that DV have been met.
+//
+//go:nosplit
 func CalculateDvMask(W [80]uint32) uint32 {
 	mask := uint32(0xFFFFFFFF)
 	mask &= (((((W[44] ^ W[45]) >> 29) & 1) - 1) | ^(DV_I_48_0_bit | DV_I_51_0_bit | DV_I_52_0_bit | DV_II_45_0_bit | DV_II_46_0_bit | DV_II_50_0_bit | DV_II_51_0_bit))
@@ -363,6 +367,7 @@ func not(x uint32) uint32 {
 	return 0
 }
 
+//go:nosplit
 func SHA1_dvs() []DvInfo {
 	return sha1_dvs
 }