Phase 1 — Foundation

Mirror NPE's dependency-free spine in Rust:

* exceptions   — NetworkError + ParsingError + ContentUnavailable
                 + ExtractionError tree, with reqwest/serde_json conversions
* localization — Localization + ContentCountry, default (en, GB)
* downloader/  — Downloader trait, Request builder, Response,
                 reqwest blocking default impl
* page         — continuation-token carrier
* image        — Image + ImageSet + ResolutionLevel
                 (HEIGHT_UNKNOWN/WIDTH_UNKNOWN = -1)
* metainfo     — title/content/url/url_text grab-bag
* service      — StreamingService trait + LinkType + ServiceInfo
* newpipe      — process-global Downloader / Localization /
                 ContentCountry singleton

Foundational invariants nailed down (per SPEC §3):
* HTTP non-2xx returns Ok(Response); only 429 throws NetworkError::Recaptcha
* Response header keys lowercase-normalized
* Request.add_header PARITY with NPE bug (silent overwrite);
  append_header is our clean addition
* default Localization is en-GB
* No cookie jar in the default downloader

Tests: 7 unit + 7 live smoke against httpbin.org (gated on
'online-tests' feature). All green.

src/image.rs

@@ -0,0 +1,72 @@
+// Image + ImageSet + ResolutionLevel. Mirrors NPE Image.java.
+//
+// HEIGHT_UNKNOWN / WIDTH_UNKNOWN are -1 sentinels per SPEC §3 invariant #10
+// — kept as i32, not Option<u32>, because several JSON output sites encode
+// this directly.
+
+pub const HEIGHT_UNKNOWN: i32 = -1;
+pub const WIDTH_UNKNOWN: i32 = -1;
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
+pub enum ResolutionLevel {
+    Low,
+    Medium,
+    High,
+    Unknown,
+}
+
+impl ResolutionLevel {
+    pub fn from_height(height: i32) -> Self {
+        if height == HEIGHT_UNKNOWN {
+            ResolutionLevel::Unknown
+        } else if height <= 175 {
+            ResolutionLevel::Low
+        } else if height <= 720 {
+            ResolutionLevel::Medium
+        } else {
+            ResolutionLevel::High
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct Image {
+    url: String,
+    height: i32,
+    width: i32,
+    estimated_resolution_level: ResolutionLevel,
+}
+
+impl Image {
+    pub fn new(
+        url: impl Into<String>,
+        height: i32,
+        width: i32,
+        estimated_resolution_level: ResolutionLevel,
+    ) -> Self {
+        Self {
+            url: url.into(),
+            height,
+            width,
+            estimated_resolution_level,
+        }
+    }
+
+    pub fn url(&self) -> &str {
+        &self.url
+    }
+
+    pub fn height(&self) -> i32 {
+        self.height
+    }
+
+    pub fn width(&self) -> i32 {
+        self.width
+    }
+
+    pub fn estimated_resolution_level(&self) -> ResolutionLevel {
+        self.estimated_resolution_level
+    }
+}
+
+pub type ImageSet = Vec<Image>;