Add layered settings, and fix a migration mechanism that silently lied

Two things, because the second could not be built on the first.

The schema stamp was part of the same execute_batch as the CREATE TABLE IF NOT
EXISTS statements, and it wrote unconditionally. Opening an older file therefore
added no columns but flipped the version forward anyway; opening a *newer* file
stamped it back down and then wrote rows the newer build could not read. Both
produced a database whose recorded version was a lie, and every future schema
change would have inherited it.

Now the version is read before anything is applied, migrations are ordered and
forward-only inside one transaction, a newer file is refused with SchemaTooNew
rather than downgraded, and a supported version raised without a step to reach
it fails at the first open instead of claiming success. Tested by stamping a
file at version 99 and asserting both the refusal and that the stamp is left
untouched.

lumbridge-settings resolves compiled default -> settings.toml -> environment.
The environment sits above the file deliberately: decision 0016 calls
LUMBRIDGE_CLAUDE_OAUTH=0 "one switch off", and a switch a config file can
silently re-enable is not a switch. A pinned value renders disabled and names
the variable, rather than accepting an edit that would do nothing.

Every field carries a WriteAuthority. Routing all writes through Configure is
the obvious design and would hand a layout-only agent the program every future
pane launches — the guarantee decision 0006 exists to make. Anything naming a
program, path or destination is Human-only, asserted by a test that reads the
path rather than trusting the author.

Four paths are permanently not settings, with the reason recorded beside each
and a test asserting their absence: the usage endpoint URL, the credentials
path, the client identity, and the shell program. A configuration file that can
redirect where an access token is sent is a credential exfiltration path with a
friendly name.

Environment access is a trait rather than std::env, because the workspace forbids
unsafe, set_var is unsafe in Rust 2024, and the layering rule has to be testable
without mutating the process running the test.

Verified live with LUMBRIDGE_CLAUDE_OAUTH=0: the account-endpoint row reads off,
greyed, "pinned by LUMBRIDGE_CLAUDE_OAUTH". The Advanced page names every file,
endpoint and child process Lumbridge touches and states that nothing is sent
anywhere else — as a fact, not as a toggle nobody can flip.

File loading, comment-preserving writes and editable controls are not in this
pass; 0022 records why that order is the honest one.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -3322,6 +3322,7 @@ dependencies = [
  "lumbridge-core",
  "lumbridge-harness",
  "lumbridge-runtime",
+ "lumbridge-settings",
  "lumbridge-storage",
  "lumbridge-terminal",
  "lumbridge-theme",
@@ -3377,12 +3378,21 @@ dependencies = [
  "thiserror 2.0.20",
 ]
 
+[[package]]
+name = "lumbridge-settings"
+version = "0.0.1"
+dependencies = [
+ "serde",
+ "toml 0.9.12+spec-1.1.0",
+]
+
 [[package]]
 name = "lumbridge-storage"
 version = "0.0.1"
 dependencies = [
  "lumbridge-core",
  "rusqlite",
+ "tempfile",
 ]
 
 [[package]]
@@ -6142,6 +6152,21 @@ dependencies = [
  "toml_edit 0.22.27",
 ]
 
+[[package]]
+name = "toml"
+version = "0.9.12+spec-1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cf92845e79fc2e2def6a5d828f0801e29a2f8acc037becc5ab08595c7d5e9863"
+dependencies = [
+ "indexmap",
+ "serde_core",
+ "serde_spanned 1.1.1",
+ "toml_datetime 0.7.5+spec-1.1.0",
+ "toml_parser",
+ "toml_writer",
+ "winnow 0.7.15",
+]
+
 [[package]]
 name = "toml"
 version = "1.1.4+spec-1.1.0"
@@ -6166,6 +6191,15 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "toml_datetime"
+version = "0.7.5+spec-1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "92e1cfed4a3038bc5a127e35a2d360f145e1f4b971b551a2ba5fd7aedf7e1347"
+dependencies = [
+ "serde_core",
+]
+
 [[package]]
 name = "toml_datetime"
 version = "1.1.1+spec-1.1.0"