Accept either-platform absolute paths and fix v2 NewSession

[foundev]

Fixes #45. Fixes #46. Refs #47.

Problem

• Client-side session path validation used filepath.IsAbs, so it only accepted paths that are absolute for the client host. A Windows client could not address a POSIX agent with cwd /, and a POSIX client could not address a Windows agent (ACP session paths are validated with the host filepath.IsAbs, so "/" is rejected on Windows #45).
• v2.Connection.NewSession(ctx, directory) passed an empty Initialization, so the convenience wrapper failed before sending session/new (v2 Connection.NewSession convenience wrapper can never succeed #46).

Result

• acpvalidate.AbsolutePath now accepts paths absolute on either platform: POSIX (/dir), Windows drive-absolute (C:\dir, C:/dir), and UNC (\\server\share). Drive-relative (C:dir) and root-relative (\dir) paths remain rejected. The v1 facade, the v2 facade, and v2/mcp use the shared check; host-local paths (clienthost, the v2 runner) keep host-native filepath.IsAbs checks.
• v2.Connection.NewSession now uses the initialization from the most recent successful Initialize call and returns a clear error if called before initialization.

Validation

• go vet ./...
• go test -race for the root, internal/acpvalidate, v2, and v2/mcp packages
• New regression tests: TestNewSessionAcceptsPOSIXAbsoluteAgentPath, TestV2NewSessionUsesStoredInitialization, TestValidatePathsAcceptsAbsolutePathsFromEitherPlatform, plus the expanded TestAbsolutePath table.

Limits

• The absolute-path check is deliberately permissive across platforms; the agent stays the authority on its own filesystem (Follow-up to #45: cover every agent-side path validator and the POSIX-to-Windows direction #47 tracks the remaining agent-side validator coverage).