Adding an option for color profiles and a side-by-side mode

CHANGES-SIDEBYSIDE.md

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+# VBinDiff — Side-by-Side & Colorized Fork
+
+Based on madsen/vbindiff (the canonical upstream). Two changes from stock
+VBinDiff:
+
+## 1. Side-by-side layout (opt-in via `-s`/`--side-by-side`)
+
+By default VBinDiff behaves exactly as before: two files stacked one above
+the other, fits an 80-column terminal.
+
+Pass `-s` to instead show the two files side by side (left/right) with a
+colored divider between them:
+
+    vbindiff file1 file2              # default: stacked, like stock vbindiff
+    vbindiff -s file1 file2           # side by side, 16 bytes/line, needs ~155 cols
+    vbindiff -s -w 8 file1 file2      # side by side, 8 bytes/line, needs ~85 cols
+
+`-w`/`--width` sets bytes per line in either mode (default 16). It matters
+most in side-by-side mode, where it's the main lever for fitting your
+terminal width — each pane needs roughly `10 + 3*width + width/8 + width`
+columns, so two panes plus a divider need about `2*that + 3`.
+
+If your terminal is too narrow for the requested width, VBinDiff exits
+with a clear error telling you the required width, rather than corrupting
+the display.
+
+## 2. Configurable colors (default matches original vbindiff)
+
+The default palette is unchanged from stock VBinDiff — the classic
+white-on-blue theme, red for differences, yellow for edits, reverse-video
+filename/mode bars. The one addition is the pane divider in side-by-side
+mode, styled to match (white on blue, bold).
+
+Colors are fully configurable via a config file, if you want to move away
+from the original look:
+
+    vbindiff -c mytheme.conf file1 file2
+
+Or drop a file at `~/.vbindiffrc` to have it load automatically every run
+(silently ignored if it doesn't exist; parse errors in an explicit `-c`
+file are reported and abort startup).
+
+See `vbindiff-example.conf` in this repo for the file format and a sample
+"classic diff" green/red theme.
+
+## Implementation notes
+
+- `lineWidth` (bytes/line) is now a runtime value instead of a compile-time
+  constant, so the file-buffer indexing was changed from a 2D array trick
+  to explicit `row*lineWidth + col` arithmetic.
+- `sideBySide` (default false) gates between the original stacked layout
+  code path and the new side-by-side one throughout `calcScreenLayout()`,
+  `initialize()`, and `positionInWin()`.
+- Screen layout (`leftMar2`, `paneWidth`, `screenWidth`) is computed from
+  `lineWidth`/`sideBySide` at startup via `computeLayoutMetrics()`.
+- Colors moved from 4 hardcoded curses color pairs to one configurable
+  `{fg, bg, attrs}` triple per UI style, loaded into curses color pairs at
+  startup and optionally overridden by a config file before that.
+- The Goto/Find popup windows position themselves relative to the left
+  pane, right pane, or centered across both in side-by-side mode; in
+  stacked mode they use the original top/bottom vertical positioning.
+- Gotcha fixed along the way: a `GetOpt` `ArgFunc` callback for a no-argument
+  flag must return `false` (not `true`) when it doesn't consume an
+  argument, or the library will swallow the next positional argument
+  (e.g. your first filename) as if it belonged to the flag.
+
+## Man page
+
+`tools/vbindiff.pod.tt` (the POD-in-Template-Toolkit source `vbindiff.1` is
+generated from) has been updated to document `-s`, `-w`, and `-c`, with a
+short "Side by side mode" and "Color configuration" section. Building it
+requires the `libtemplate-perl` package (`Template.pm`); if that's not
+installed, `make` still builds the `vbindiff` binary fine and just skips
+the man page.
+
+## Build
+
+Same as upstream:
+
+    git submodule update --init --recursive   # fetches GetOpt dependency
+    autoreconf -i
+    ./configure
+    make