new memory and project layout

apps/README.md

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+# ROM applications
+
+Applications in this directory execute directly from ROM; the reset bootstrap
+does not copy them. Each application must have its own non-overlapping `ORG`
+below `8000` and should include `bios.inc` from the project's `include`
+directory instead of accessing the UART hardware directly.
+
+The current allocation is:
+
+- `1000-180C`: Tiny BASIC
+- `180D-7FFF`: available for more applications (`1900` is the next convenient
+  aligned origin)
+
+The resident BIOS and monitor own high RAM at `C000-FFFF`. Tiny BASIC owns
+`8000-BFFF` for its workspace and stack. A future application may reuse Tiny
+BASIC's lower-RAM allocation because ROM applications run one at a time, but it
+must leave `C000-FFFF` untouched.
+
+Applications using the interrupt-driven console must leave interrupt mode 2,
+the `I` register, and the BIOS UART/CTC configuration intact. They may return
+normally if they preserve the caller's stack, or jump to `BIOS_MONITOR` for a
+non-returning monitor warm start after replacing the stack pointer.
+
+To add an application, add its assembler target and ROM offset to the Makefile,
+overlay its binary in the `monitor.prog.bin` recipe, and add a monitor command
+or use `G address` to launch it.