feat: add Decoder.UseNumber and the Number type

Assisted-by: GLM 5.3 Flash

docs/API.md

@@ -157,7 +157,9 @@ and every 64 fields during the reflection walk.
 
 When decoding into a struct, these values convert onto the destination's
 concrete types: any integer or unsigned width, floats, slices, nested structs
-and `map[string]T`.
+and `map[string]T`. `Decoder.UseNumber` replaces the two numeric rows of the
+table with `Number`, which keeps the literal; see
+[Numbers as literals](#numbers-as-literals).
 
 ### Target constraints
 
@@ -211,6 +213,29 @@ The decoder converts to the destination type with explicit overflow checks:
 A conversion that the rules do not allow produces an error wrapped with the
 offending key or index, for example `p: interpres: integer 300 overflows uint8`.
 
+### Numbers as literals
+
+`NewDecoder().UseNumber()` decodes every integer and float into `Number`, a
+string type that carries the literal the document wrote: `0x1f`, `1_000`,
+`+1.0`, `inf`. The shape is validated as strictly as ever, so `01` and `1__0`
+remain parse errors; only the evaluated value is replaced by the literal. A
+round trip through the value tree and `Marshal` keeps the spelling, where the
+default tree normalises `0x1f` to `31` and `+1.0` to `1.0`.
+
+```go
+var tree map[string]any
+err := interpres.NewDecoder().UseNumber().Decode(data, &tree)
+lit := tree["rate"].(interpres.Number) // "1_000"
+```
+
+A destination of a concrete kind is unaffected: an `int64` field, a `float64`
+field and a `time.Duration` field take the evaluated value they always took,
+and a `Number` field takes the literal. `Number.Float64` and `Number.Int64`
+evaluate the literal on demand, with an error for a float asked as an integer
+and for a literal that is not a valid TOML number. `Marshal` writes a `Number`
+as its bare literal and rejects one that is not a valid TOML number, whether it
+stands alone or inside a value array.
+
 ### Date-time values
 
 Offset date-times decode into `OffsetDateTime`, whose embedded `time.Time` is the
@@ -639,6 +664,7 @@ concurrent use.
 | Method | Default | Effect |
 |---|---|---|
 | `DisallowUnknownFields()` | off | a key with no matching struct field is an error |
+| `UseNumber()` | off | integers and floats decode into `Number`, which carries the literal; see [Numbers as literals](#numbers-as-literals) |
 | `MaxDepth(depth int)` | `10000` | bound how deeply arrays and inline tables may nest |
 | `MaxInputSize(size int)` | no limit | bound the size of the document, in bytes |
 
@@ -688,6 +714,12 @@ See [Custom encoding](#custom-encoding-marshaler).
 
 See [Custom decoding](#custom-decoding-unmarshaler).
 
+### `type Number string`
+
+The literal a number was written with, what `UseNumber` decodes into and what
+`Marshal` writes back as it is. See
+[Numbers as literals](#numbers-as-literals).
+
 ### Date-time wrappers
 
 ```go