sl0ppty: say which build you are talking to

The middle of the status line is empty unless the pointer is on something,
which is most of the time. It says which sl0ppty this is now.

That reads as decoration and is not. A session keeps the binary it started
with, so after a rebuild there are two versions in play: the one on disk and
the one you are attached to. This session has been dogfooding sl0ppty for a
while, and "which build is this?" has already been the answer to two bugs
that looked like something else -- once when a config reload appeared to do
nothing, and once when kitty graphics were fixed and still broken on screen.
The banner answers it without asking, and `sl0ppty --version` answers for the
binary, from the same string.

The version is baked in by the Makefile from `git describe --always --dirty`,
into a generated header that is only rewritten when the value changes -- so a
commit rebuilds and a second `make` does nothing.

It takes the hint's slot and yields to it: a hint is about what you are doing
right now, the banner is about always, and it comes back the moment the
pointer moves off. Both give way to the ends of the line, which was already
the rule for hints and is why a narrow terminal loses the middle rather than
its session name. `version_banner false` for anyone who finds it noisy; it is
a separate knob from `hints`, and the tests hold both directions of that.

1 files changed, 17 insertions, 2 deletions

diff --git a/Makefile b/Makefile
index 1b95a57..b1556b0 100644
--- a/Makefile
+++ b/Makefile
@@ -14,8 +14,14 @@ VT_INC   := $(VT_OUT)/include
 # header rebuilds what actually uses it. Without this a header edit rebuilt
 # nothing and the only safe move was `make clean`, which is how a 0.1s build
 # turns into a habit of throwing the whole thing away.
+# The version is baked in, because the first question when a session is
+# behaving oddly is "which build is this?" -- and a session keeps the binary it
+# started with, so the answer is not "whatever is in build/ now".
+VERSION  := 0.1.0
+GITDESC  := $(shell git describe --always --dirty 2>/dev/null || echo unknown)
+
 CFLAGS   := -std=c23 -O1 -g -Wall -Wextra -Wno-unused-parameter -MMD -MP \
-            -I$(VT_INC) -Isrc
+            -I$(VT_INC) -Isrc -Ibuild
 LDFLAGS  :=
 
 SRC      := $(wildcard src/*.c)
@@ -44,9 +50,18 @@ all: $(BIN) ## build sl0ppty
 $(BIN): $(OBJ) $(VT_LIB)
 	$(CC) $(OBJ) $(VT_LIB) -o $@ $(LDFLAGS)
 
-build/%.o: src/%.c $(VT_LIB) | build
+build/%.o: src/%.c build/version.h $(VT_LIB) | build
 	$(CC) $(CFLAGS) -c $< -o $@
 
+# Rewritten only when the value actually changes, so `make` after a commit
+# rebuilds, and `make` twice in a row does not.
+.PHONY: FORCE
+build/version.h: FORCE | build
+	@printf '/* generated by the Makefile */\n#define SL0PPTY_VERSION "%s+%s"\n' \
+	  '$(VERSION)' '$(GITDESC)' > $@.tmp
+	@cmp -s $@.tmp $@ || mv $@.tmp $@
+	@rm -f $@.tmp
+
 build:
 	@mkdir -p build