Document converter validation and deferred variable resolution

converters.md:
- Update Boolean accepted values to include 1/0 and note rejection
- Add error examples for built-in converters (integer, boolean, long)

options.md:
- Update variable resolution description: re-resolved on each parse
  cycle (#520), testable with env var changes after construction
- Note env var priority over DefaultValueProvider (#521)

Author: stalep

content/docs/aesh/converters.md

@@ -26,7 +26,7 @@ The `ConverterInvocation` provides:
 - `String`
 - `int` / `Integer`, `long` / `Long`, `short` / `Short`, `byte` / `Byte`
 - `float` / `Float`, `double` / `Double`
-- `boolean` / `Boolean` (accepts `true`, `false`, `yes`, `no`)
+- `boolean` / `Boolean` (accepts `true`/`false`, `yes`/`no`, `1`/`0` — case-insensitive; rejects unrecognized values)
 - `char` / `Character`
 - `File`, `Path`, [`Resource`](../resources)
 - Any `Enum` type (matched by name, case-insensitive)
@@ -128,13 +128,26 @@ Timeout set to 300 seconds
 
 ## Error Handling
 
-When conversion fails, throw `OptionValidatorException` with a message describing what went wrong and what the user should provide instead. Æsh displays the message to the user and aborts command parsing:
+When conversion fails, throw `OptionValidatorException` with a message describing what went wrong and what the user should provide instead. Aesh displays the message to the user and aborts command parsing:
 
 ```
 $ timeout --duration abc
 Invalid duration: abc. Use format like: 30s, 5m, 2h, 1d
 ```
 
+All built-in converters validate their input and throw `OptionValidatorException` with descriptive messages:
+
+```
+$ myapp --port abc
+Invalid integer value: 'abc'
+
+$ myapp --fresh=foobar
+Invalid boolean value: 'foobar'. Expected: true/false, yes/no, 1/0
+
+$ myapp --size 999999999999999999999
+Invalid long value: '999999999999999999999'
+```
+
 ## Converter vs Validator
 
 - **Converters** transform the input string into a typed value. They answer: *"What does this string mean?"*

content/docs/aesh/options.md

@@ -952,7 +952,9 @@ private String literal;
 | `${a:-${b:-c}}` | Nested fallback chain | `${env:A:-${sys:B:-default}}` |
 | `$${...}` | Literal (no expansion) | `$${NOT_EXPANDED}` |
 
-Variable resolution applies to `defaultValue` and `fallbackValue` annotation attributes. It runs at option construction time, before `DefaultValueProvider` (which runs at parse time and takes priority over static defaults).
+Variable resolution applies to `defaultValue` and `fallbackValue` annotation attributes. Variables are resolved initially at option construction time and **re-resolved on each parse cycle**, so environment variable changes between command invocations are picked up automatically. This makes `${env:...}` defaults fully testable — tests that set env vars after command construction will see the updated values.
+
+When a `${env:...}` or `${sys:...}` variable resolves successfully, it takes priority over `DefaultValueProvider` (see [Value Resolution Chain](#value-resolution-chain)). When the variable is not set, the provider gets a chance to supply a config-based default.
 
 ### Combined Example