stabilize build system: depends, installer, boost/bdb fixes, cross targets groundwork

docs/installer.md

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+# Ubuntu Installer
+
+`installer/agrarian-installer.sh` provides a Ubuntu-only CLI for common build actions.
+
+## Usage
+
+```bash
+installer/agrarian-installer.sh [options]
+```
+
+Options:
+
+- `--host <triplet>`: target host triplet (default: `x86_64-pc-linux-gnu`)
+- `--action depends|daemon|qt|all`: build action (default: `all`)
+- `--wallet 0|1`: disable/enable wallet-related build flags (default: `1`)
+- `--jobs <n>`: parallel jobs (default: `nproc`)
+- `--update`: run `git pull --rebase` before build
+- `--reset-depends`: remove `depends/work`, `depends/built`, and `depends/<host>`
+- `--yes`: skip confirmation prompts
+- `--log <path>`: write a build log to the provided path
+
+Examples:
+
+```bash
+installer/agrarian-installer.sh --action depends --host x86_64-pc-linux-gnu --yes
+installer/agrarian-installer.sh --action all --wallet 0 --jobs 4 --log /tmp/agrarian-install.log
+```
+
+If wallet is enabled (default), installer preflight checks require:
+
+- `depends/<host>/include/db_cxx.h`
+- `depends/<host>/lib/libboost_thread*.a`
+- `depends/<host>/lib/libboost_system*.a`
+
+When missing, the installer exits early and prints the exact missing path(s) plus the `make -C depends ...` command to fix them.
+
+## Tests
+
+Run installer tests with:
+
+```bash
+installer/tests/run.sh
+```