fix(metassr-build): replace panicking Manifest::from() with proper error handling

[abhicodes0324]

Problem

Manifest::from() called .unwrap() twice with no error context:

let content = read_to_string(path).unwrap();  // panics if file missing
serde_json::from_str(&content).unwrap()        // panics if JSON malformed

This causes two real user-facing problems:

1. Running metassr run without building first - manifest.json doesn't exist yet, so the server panics immediately with a cryptic thread panic instead of a clear message.

2. Corrupted or partial build - if manifest.json exists but is malformed, the server panics with no indication of what went wrong.

Additionally, page.rs called manifest.get(route).unwrap() which panics if a route exists in the server but not in the manifest.

Changes

manifest.rs

• Removed panicking From<&S> impl
• Added Manifest::load() returning Result<Self> with actionable errors:
  • Missing file → "Could not read manifest.json at ...: Did you run metassr build first?"
  • Malformed JSON → "manifest.json is malformed: ..."

renderer/page.rs

• Manifest::from(path) → Manifest::load(path)?
• manifest.get(route).unwrap() → .ok_or_else(|| anyhow!(...))?

Before / After

# Before — cryptic panic with no context thread 'main' panicked at 'called `Option::unwrap()` on a `None` value'

# After — actionable error message
Error: Could not read manifest.json at "dist/manifest.json": No such file or directory. Did you run `metassr build` first?

Note

Could not verify output locally due to the MetaCall native dependency requirement. The correctness of this fix can be verified by code review - Manifest::load() uses map_err with ? propagation instead of .unwrap(), making panics structurally impossible for both the missing file and malformed JSON cases.