Improving block file change detection

[jtyr]

Summary

Fixes two related issues with editor macro scripts and the block file.

Problem 1: Block file not saved before macro execution

edit_block_process_cmd() runs a numbered macro script that may reference %b (the block file). However, the selected block is not written to the block file before the script runs, so %b points to a stale or nonexistent file.

The fix is to save the selected block to EDIT_HOME_BLOCK_FILE at the start of edit_block_process_cmd(), before calling edit_user_menu().

Problem 2: Block file modification missed due to second-precision timestamps

edit_user_menu() detects whether a macro script modified the block file by comparing st_size and st_mtime before and after execution. Scripts that rearrange bytes without changing file size (e.g., word-wrap replacing spaces with newlines) and complete within the same second produce identical st_size and st_mtime, causing a false negative - the editor does not reload the modified block.

The fix is to use nanosecond-precision st_mtim (via HAVE_STRUCT_STAT_ST_MTIM, already detected by configure) instead of second-precision st_mtime. Falls back to st_mtime on platforms without nanosecond support.

Test plan

• Select a block, run a macro that transforms it (e.g., sort, word-wrap) - the result should be inserted back into the editor
• Run a macro that produces identical file size but different content - the change should be detected
• Build on a platform without st_mtim - should compile and fall back to second-precision comparison